Special Equipment Needed: Gel food coloring; set of nine round cookie cutters in graduated sizes; piping bags; an Ateco piping tip #849 (or any large star tip); extra-tall cake scraper (14" at least-you can also use a quilting ruler!); a candy thermometer. Bake the Gingerbread Cake: Preheat your oven to 350°. Prepare four 6"x3" pans by spraying the bottoms with cooking spray and laying a baking paper round at the bottom of each. Combine your dry ingredients in a medium bowl and set aside. Place your butter, white sugar, and brown sugar in the bowl of a stand mixer fitted with the paddle attachment and beat on medium speed until light and fluffy. Add your eggs in one at a time until all six are incorporated, then stream in your molasses and add the vanilla, beating another 30 seconds to make sure everything is homogenous. Scrape the bottom of the bowl, then add your dry ingredients in three additions alternating with your milk, starting and ending with the dry mix. Stop the mixer as soon as you see no more streaks in the batter. Divide your batter between the four prepared pans and bake for 45-55 minutes-you'll know the cakes are baked through when you press gently on the top and the cake springs back. A general note about this batter: we're using a lot of liquid here, which keeps the cake super moist and yummy. This can cause the cake to bake up with a slight dip in the top instead of a dome. Don't worry if you see that happen, just trim the topmost edges of the cake away when you're getting ready to cut your layers, and when you are cutting circles out of the middle of your layers you can use the larger cookie cutters to cut out any caramelization or even holes in the layers left from the top! Remove the cakes from the oven and cool on a rack for at least 20 minutes before using a small paring knife to free the sides from the pan and removing the cake. Allow to come to room temperature, then wrap in plastic and allow to chill all the way through in the refrigerator before leveling and torting, ideally for at least 2 hours. Bake the Green Eggnog Cake: Preheat your oven to 350°. Prepare two 6"x3" pans by spraying the bottoms with cooking spray and laying a baking paper round at the bottom of each. Combine your dry ingredients in a medium bowl and set aside. Place your butter and sugar in the bowl of a stand mixer fitted with the paddle attachment and beat on medium speed until light and fluffy. Add your eggs in one at a time until all four are incorporated, then add the vanilla, beating another 30 seconds to make sure everything is homogenous. Combine your eggnog with your green food coloring gel and set aside. Scrape the bottom of the bowl, then add your dry ingredients in three additions alternating with your eggnog, starting and ending with the dry mix. Stop the mixer as soon as you see no more streaks in the batter. Divide your batter between the two prepared pans and bake for 40-50 minutes-you'll know the cakes are baked through when you press gently on the top and the cake springs back. Remove the cakes from the oven and cool on a rack for at least 20 minutes before using a small paring knife to free the sides from the pan and removing the cake. Allow to come to room temperature, then wrap in plastic and allow to chill all the way through in the refrigerator before leveling and torting, ideally for at least 2 hours. Make the Buttercream: Place the sugar, brown sugar, and water in a medium saucepan and set over medium heat, stirring occasionally. Use a pastry brush dipped in water to melt the sugar crystals that form on the sides of the pan. When the syrup begins to boil, reduce the heat to low and insert a candy thermometer into the syrup. Allow it to continue to boil while moving on to the next step. Place the egg whites in the clean bowl of a stand mixer fitted with a whisk attachment. Whip the whites until frothy, then sprinkle in the cream of tartar (this improves the strength of the meringue). Keeping an eye on the temperature of the sugar syrup, increase the speed of the mixer to high to achieve stiff peaks. Ideally, you will time this to coincide with the sugar syrup reaching 248°-250° F, the Firm Ball Stage. If your whites reach stiff peaks before the sugar syrup reaches the correct temperature, reduce the mixer speed to the lowest setting and leave running until the syrup is ready. When the syrup reaches 248°-250° F, increase the mixer speed to the highest setting. Remove the saucepan from the heat and slowly pour the hot syrup down the side of the bowl into the egg whites, being very careful to avoid allowing the stream to hit the whisk and spatter. Pour slowly and steadily until all the syrup has been added, and continue beating the meringue until it becomes light and fluffy and holds its shape exactly when the beater is removed, roughly 3-5 minutes. After this time, the meringue may still be hot-if so, place the bowl in the refrigerator (or freezer) for 8-10 minutes until it has cooled all the way through. When the meringue has cooled, place the bowl back on the mixer, replacing the whisk with the paddle attachment. Begin mixing on low, and add the butter 1/4 cup (half a stick) at a time, beating well after each addition. Once the butter is added, increase the speed to medium and beat the buttercream until it becomes light and fluffy. It may look soupy or curdled shortly after all the butter is added-this is okay! Just keep on beating it and it will whip up into a light, fluffy cloud. While the buttercream is whipping up, mix your cream cheese, sour cream, vanilla, and salt together in a small bowl. Add to your buttercream all at once and beat on low speed until incorporated. Cut Your Layers: Level and torte each of the 4 gingerbread cakes into 3 layers for a total of 12. Level and torte your green eggnog cake into 3 layers each for a total of 6, trying to keep these layers the same thickness as the gingerbread layers. Using your 9 round cutters, cut one circle for each cutter size out of the middle of 8 of your gingerbread cakes, reserving the other 3. Then, using the second-smallest cutter, cut a round out of the middle of 2 more gingerbread rounds (these layers will create the trunk of your tree), leaving just 1 last layer whole. Discard the small rounds you've removed (read: snack away!). Use the 8 larger cutters to cut green rounds out of the green eggnog layers, cutting 2 small rounds out of each layer once you get to the 4 smaller sizes to economize on space. You should wind up with 8 total green rounds of varying sizes. This time, reserve the small rounds, and snack on the scraps left from the layers they were cut from. Take the cutter that's two sizes smaller and place it gently on top of each green circle. Press lightly to mark the top of each green circle, and using that as a guide, gently trim the edges of each round at an angle. Insert each angled green round into its properly-sized gingerbread layer to keep everything organized, and to prepare for building the cake, arrange your stack of layers stack so the gingerbread layer with the smallest, empty hole goes at the bottom, and then the layers should go from smallest to biggest green round, the opposite of how it will appear when it's finally stacked with buttercream to look like a tree. Top it off with the 2 "trunk" layers, and finally the single whole layer. Wrap the stack in plastic and refrigerate until you're ready to use it. Color Your Buttercream: Color two cups of your holiday buttercream with 3-4 drops of your forest green gel color. Color half a cup each with 1/2 teaspoon red, 3 drops yellow, and 5-6 drops blue. Mix 1/4 cup with a few drops of brown for a tree trunk color. Mix 1.5 cups with 1/4 tsp deep pink, and red until the color is as bright as you want it! It's Dr. Seuss, so go a little nuts. Place each color in its own piping bag with no tip; you'll cut the tips away as needed to create workable holes. Tip: If the buttercream looks broken at any point, microwave for 3 seconds at a time once or twice, mixing by hand each time to re-incorporate. Stack Your Cake: Place a small dollop of buttercream on the cake plate or drum you're using as a base and center the single whole cake layer on it. Spread a thin layer of plain, un-colored buttercream on the layer and then place one of the 2 "trunk" layers down. Add another layer of plain buttercream, this time icing around the hole cut out of the middle of the slice. Stack the second "trunk" layer on top, use plain buttercream to add a layer of filling, then pipe your brown buttercream into the hole created by those two layers. Fill it a little above the top, then level it with your spatula. Now, begin to add your green tree layers. Starting with the layer with the largest green insert, place it on top of the "trunk" layers with the angle you've cut into the edges sloping down. Pipe a ring of plain buttercream around that sloped cake to seal it, then smooth it, trying not to get any plain buttercream on the green part of the cake. Now you'll add your Christmas lights! Cut off a very small bit of the tip of your red, blue, green, and yellow piping bags to leave a small hole, and pipe concentric rings of color on top of the green part of the layer, switching up colors as you go and leaving space between each ring. Fill in the gaps with your green, and very gently smooth that section of buttercream down. Add plain buttercream to the gingerbread portion of the layer and smooth to create one level layer of buttercream, then place your next largest green layer on top. Continue this process until you run out of green layers. You'll be left with a gingerbread layer with the smallest hole cut into it, but no green cake inside. Fill this gap with yellow buttercream to represent the start at the top of the tree, and smooth out. Ice over the top with a very thin layer of plain buttercream to seal that yellow in, then refrigerate the stacked cake. Ice Your Cake: Once your cake has had 15-20 minutes to chill, trim the sides if you'd like. If desired, you can also start icing the cake by doing a light coat of plain, un-colored buttercream over the whole cake so that when you apply your colored buttercream you don't have to use very much. Using your leftover blue (perhaps cutting a bigger hole in the tip of your piping bag) and your 1.5 cups pinky red, pipe alternating rings of buttercream around the sides of the cake, keeping in mind that you have more red than blue so keep the red sections about 2-3 times taller. It doesn't need to be, and in fact shouldn't be, perfect, since a little unevenness is in keeping with Dr. Seuss' aesthetic! Pipe a large dollop of red on the top of the cake and, using a small offset spatula, smooth it down. Using a very tall straight edge (you can use a quilting ruler if you don't have a cake scraper tall enough!), smooth down the rings of color on your cake, keeping the scraper level so you don't muddy up your layers of color. When you're happy with that, clean up the top of your cake if you need to and refrigerate. Finishing Touches: Use an Ateco tip 849 (or any large star tip) and any leftover green buttercream to pipe ruffled puffs around the top of the cake, making pretty buttercream "trees." Pipe a little yellow dot at the top of each tree. You can also finish the bottom edge of the cake with a little yellow pearl border like I did, or leave it, your call! Get creative! Now you can slice your Seussian masterpiece to reveal your sneaky surprise inside, a stunning Christmas tree that will warm the heart of even the grumpiest of grinches this holiday season!