Preheat oven to 350 degrees. Butter two 8-by- 12-inch cake pans, and line with parchment paper. Butter parchment, and lightly dust with cocoa powder; set aside. Sift cocoa powder, flour, baking powder, and salt into a medium bowl; set aside.
Put butter and sugar into the bowl of an electric mixer fitted with the paddle attachment; mix on medium speed until pale and fluffy, 3 to 4 minutes. Mix in eggs, 1 at a time, and vanilla. Reduce speed to low. Add cocoa mixture in 2 batches, alternating with the buttermilk.
Divide batter evenly between pans. Bake until a cake tester inserted into centers comes out clean, about 35 minutes. Let cool on a wire rack 15 minutes. Invert cakes onto rack; remove parchment, and reinvert. Let cool completely.
Trim cakes level. Place 1 cake on a platter. Spread with 1 cup frosting. Top with remaining cake. Spread 4 1/2 cups frosting on top and sides. Fill a pastry bag fitted with a petal tip (such as Ateco #126) with remaining frosting. Pipe a ruffled border along edges. Cake can be refrigerated up to 1 day. Serve with scrolled tuiles.