To Make the Shells: Line 2 baking sheets with parchment paper. Pulse almond flour and powdered sugar in a food processor until well combined. Sift using a mesh sieve into a small bowl and set aside. In a stainless steel bowl, whisk egg whites at medium speed until frothy, about 1 minute, then add cream of tartar. Continue to whisk until soft peaks form, about 2 minutes, then turn down to low speed. Add superfine sugar and whisk at high speed until stiff peaks form, about 5 minutes. Peaks should not be able to fall and mixture should not move when bowl is turned upside down. Sift almond flour mixture again into bowl of egg whites. Using a spatula, fold almond mixture and egg whites together. The mixture seems very dry initially until the whites start to mix. Fold until mixture looks shiny and smooth and batter drips slowly from the spatula but only keeps a shape for about 15 seconds (peak disappears). Transfer batter into a pastry bag with a 1/2" round tip or a freezer bag with one corner cut to fit the tip. A well folded batter should start to ooze out of tip when the pastry bag is filled. Pipe 3/4" circles separated by 1". For best results, print out 'macaron templates' found online and place under parchment paper as a guide and remove before baking. When piping, keep the tip in the middle and wiggle around while piping to fill the circle, stopping just before reaching the edge of the circle and lifting the tip from the side. After completing the piping, tap baking sheet a few times onto counter or table to release air pockets and let rest for 10 minutes. While batter is resting, preheat oven to 375 degrees and place another empty baking sheet onto middle rack. Once oven reaches 375 and batter has rested, decrease temperature to 325 and place first baking sheet onto empty pan in oven. Prop open oven door slightly using an oven mitt and bake for 7 minutes then turn pan to bake evenly, another 7 minutes. When all techniques are executed well, the shells should bake with a frilly "foot.". Remove from oven if tops of shells are crisp, not soft, and can be slightly lifted without separating or completely sticking to the pan when checked. Bake another minute or two until shells are crisp but not browned. Let sheet cool on wire rack for 2 minutes. If baking a second sheet, reheat oven to 375 degrees before reducing back to 325 degrees and baking as before with door propped. Remove shells to wire rack to cool completely. To Make Filling: Beat softened butter and sugar until well incorporated. Add vanilla and beat until smooth. To Assemble Macarons: Match shells into pairs according to similar sizes/shapes. Transfer filling into a pastry bag with small round tip or into a sandwich bag with a small corner cut. Pipe a 1" mound onto the flat side of one shell. Take empty shell half and sandwich onto the other with flat sides facing, pushing filling down until it meets the edge of the two shells all around. Although macarons can be eaten immediately, they taste best after 24 hours refrigerated then brought back to room temperature before enjoying. Flavor Alternatives: For Coffee: add 1 tbsp espresso powder and 1 tbsp cocoa powder to flour mixture. Replace 1 tsp espresso powder diluted with a little water into filling instead of vanilla. For Salted Caramel: add 1 tbsp cocoa powder to flour mixture and sprinkle coarse salt on tops of batter after baking sheets are tapped to release air. Fill with your favorite caramel frosting recipe with added salt. For Chocolate: replace 1/4 cup of the almond flour with 3 tbsp cocoa powder. Fill with your favorite chocolate ganache recipe. For Vanilla Bean: add 1/2 of a scraped bean to egg whites after stiff peaks formed and before sifting flour onto whites. Add 1/2 of a scraped bean into the filling mix when beating. For Pistachio: replace 1/4 cup of the almond flour with 1/4 cup of ground pistachios. Add 2 tbsp of ground pistachios to filling when beating. For Green Tea: add 1 tbsp of green tea powder to flour mixture. Add 2 tsp of green tea powder to the filling when beating. For Cheesecake: replace 1/4 cup of the almond flour with 1/4 cup of ground graham crackers. Fill with your favorite cheesecake buttercream recipe.