Step 1Preheat oven to 350°. Line 2 (8″) round cake pans with parchment and grease with cooking spray. In a large bowl, using a handheld mixer (or in the large bowl of a stand mixer fitted with the whisk attachment), beat butter and sugar on medium-high speed until light and fluffy. Add eggs, one at a time, beating to blend after each addition. Beat in vanilla.
Step 2In a medium large bowl, whisk flour, cocoa powder, baking soda, and salt. Add half of dry ingredients to egg mixture. Beat on medium-low speed until just combined. Add buttermilk, vinegar, and red food coloring and beat on medium speed until combined. Add remaining dry ingredients and beat on medium-low speed until just combined. Divide batter between prepared pans.
Step 3Bake cakes until a tester inserted into the center comes out clean, 25 to 30 minutes. Let cool 15 minutes, then invert onto a wire rack and let cool completely.
Step 4Using a serrated knife, level tops of cakes, reserving scraps for decorating. Place one layer on a platter, then spread top with a thick layer of frosting. Place second cake layer on top. Frost top and sides of cake. Crumble reserved cake pieces on top of cake, lightly pressing to adhere.
