Preheat oven to 350 degrees. Butter two 8-inch round cake pans (2 inches deep); dust with cocoa. Sift cocoa, flour, sugar, baking soda, baking powder, and salt into the bowl of a mixer. Beat on low speed until just combined. Raise speed to medium, and add eggs, buttermilk, water, oil, and vanilla. Beat until smooth, about 3 minutes.
Divide batter between pans. Bake until set and a toothpick inserted into the centers comes out clean, about 35 minutes. Let cool for 15 minutes. Turn out from pans. Transfer, faceup, to wire racks. Let cool completely.
Spread 2 cups chocolate frosting onto top of 1 cooled layer. Top with remaining layer; frost top and sides with remaining 2 cups frosting.
Unfrosted cake layers can be stored, wrapped in plastic, at room temperature or refrigerated overnight. Frosted cake can be refrigerated overnight.