Preheat oven to 350F/170C.
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.