1. Preheat oven to 350 degrees. Butter and flour two 8-by-2-inch cake pans, tapping out excess flour. In a medium bowl, whisk flour, cocoa powder, baking powder, baking soda, and salt.

2. In a large bowl, using an electric mixer, beat butter and sugar until light and fluffy. With mixer on low, beat in eggs and yolks, one at a time. Beat in vanilla. Alternately beat in flour mixture and buttermilk, beginning and ending with flour mixture; mix just until combined.

3. Divide batter between pans; smooth tops. Bake until cakes pull away from sides of pans, 32 to 35 minutes. Let cool in pans 10 minutes. Run a knife around edges of pans and invert cakes onto a wire rack. Let cool completely.

4. Place one cake, bottom side up, on a cake stand. Tuck strips of parchment paper underneath. Using an offset-spatula or table knife, spread top with Whipped Frosting. Top with remaining cake; frost top, then sides.