Glorious Red, White, and Blue Cake

Ingredients:
2 1/2 cups sifted cake flour, not self-rising (sift before measuring)
"1/4 cup unsweetened natural cocoa (such as Hersheys; not Dutch process or extra-dark cocoa)", 1 teaspoon baking soda
3/4 teaspoon salt
1 stick (1/2 cup) unsalted butter, softened
1 1/2 cups granulated sugar
1 cup vegetable oil
2 large eggs
1 (1-ounce) bottle (2 tablespoons) red food coloring
1 1/2 teaspoons white or cider vinegar
1 1/2 teaspoons vanilla extract
3/4 cup sour cream mixed with 1/4 cup water
1 1/3 cups sifted cake flour, not self-rising (sift before measuring)
1/2 teaspoon baking soda
1/4 teaspoon salt
3 large egg whites
1/4 cup sour cream mixed with 2 tablespoons water
5 tablespoons unsalted butter, softened
3/4 cup granulated sugar
1/3 cup vegetable oil
1 tablespoon grated lemon zest (best done with Microplane zester or grater)
1 teaspoon vanilla extract
2 1/2 (8-ounce) packages cream cheese (20 ounces total), softened
2 1/2 sticks (1 1/4 cups) unsalted butter, softened
2 1/2 tablespoons fresh lemon juice
2 teaspoons vanilla extract
1/4 teaspoon salt
"3 1/4 cups confectioners sugar", 1 pint (8 ounces) strawberries, preferably small; halve or quarter if large
1 pint (12 ounces) blueberries
1/2 pint (6 ounces) blackberries
2 or 3 (9-inch) round cake pans
large sifter or sieve
electric stand mixer (optional)
pastry brush
offset metal spatula (for frosting cake)

Instructions:
Heat oven to 350°F. Butter two 9-inch round cake pans, then line bottoms with rounds of wax paper. Butter paper and dust pans with some additional flour, knocking out excess.
Resift cake flour together with cocoa, baking soda, and salt into a bowl.
Beat butter and sugar in a large bowl with an electric mixer (fitted with paddle attachment if using stand mixer) at medium-high speed until combined well, then add oil and beat until very pale and creamy, about 3 minutes.
Add eggs, one at a time, beating well after each addition, and beat in food coloring, vinegar, and vanilla extract.
Reduce speed to low and mix in flour mixture and sour cream mixture in batches, beginning and ending with flour mixture, and mixing until batter is smooth.
Divide batter between pans (about 3 cups in each) and smooth tops. Rap pans on counter a couple of times to expel any air bubbles. Bake in middle of oven until a wooden pick comes out clean and cake begins to pull away from sides of pans, 25 to 30 minutes. Keep oven on.
Cool layers in pans on racks 10 minutes, then run a thin knife around edge of pans. Invert layers onto racks and peel off wax paper. Cool layers completely.
Butter one 9-inch round cake pan, then line bottom with wax paper. Butter paper and dust pan with some additional flour, knocking out excess.
Resift cake flour together with baking soda and salt into a bowl. Whisk egg whites in another bowl with sour cream mixture until combined.
Beat butter and sugar in a large bowl with an electric mixer (fitted with paddle attachment if using stand mixer) at medium-high speed until combined well, then add oil and beat until very pale and creamy, about 3 minutes. Beat in zest and vanilla extract.
Reduce speed to low and mix in flour mixture and sour cream mixture in batches, beginning and ending with flour mixture and mixing until batter is smooth. Transfer batter to pan and spread evenly. Rap pan on counter a couple of times to expel any air bubbles.
Bake in middle of oven until a wooden pick comes out clean and cake begins to pull away from side of pan, 25 to 30 minutes. Cool in pan on a rack 10 minutes, then run a thin knife around edge of pan. Invert layer onto rack and peel off wax paper. Cool layer completely.
Beat together cream cheese and butter in a large bowl with an electric mixture until creamy and smooth. Beat in lemon juice, vanilla extract, and salt.
Sift confectioners' sugar into a bowl and add to cream cheese mixture. Mix on low speed until incorporated, then increase speed to medium-high and beat until smooth.
Brush loose crumbs from cake layers with a pastry brush.
Put a red velvet layer, bottom side up, on a cake plate or stand. Spread 1 cup of frosting over layer. Cover with white layer, bottom side up, and spread with another cup of frosting. Top with second red velvet layer, bottom side up.
Spread a thin layer of frosting around sides and over top of cake. (This is called crumb-coating. It tamps down any loose crumbs to keep them out of the top layer of frosting and fills in any crevices. This is particularly important with red velvet cake layers.) Chill or briefly freeze cake just until frosting is firm.
Spread sides and top of cake with remaining frosting. Chill or briefly freeze cake just until frosting is firm.
Toss together fruits in a bowl. Let cake come to room temperature before serving and top with about 1 1/2 cups fruit. Serve remaining fruit on the side.