Preheat the oven to 350 degrees, making sure the baking rack is in the middle of the oven. Prepare two round 9-inch cake pans by cutting out a piece of parchment or wax paper to line the bottom of the pan. Grease the pans, place the parchment or wax paper in the bottom of the pan and lightly grease again. Dust the pans with flour (or cocoa powder if you don’t want the white dusting on the finished cakes). Set the pans aside.
Using a fine mesh strainer, sift together the cocoa, flour, sugar, baking soda, baking powder, and salt into a large bowl. Add the eggs, yolk, warm water, buttermilk, oil and vanilla. Mix on low speed (with a handheld mixer or in the bowl of an electric stand mixer) until smooth, about 3 minutes.
Divide the batter evenly between the prepared pans. Bake the cakes for about 32-35 minutes, until a toothpick inserted into the middle comes out clean or with moist crumbs. Do not overbake! Remove the pans from the oven and set the pans on a wire rack to cool for 15 minutes. Gently run a thin knife around the edges of the pans and unmold the cakes, removing the parchment paper liners from the bottom of the cakes. Let them cool completely, top sides ups, on a wire rack.
Trim the tops of the cake layers with a long serrated knife to make them level. Frost as desired (I highly recommend this life-changing frosting).
Update (10/6/10): several of you have mentioned that you had issues with this cake spilling over the edges of the cake pan while baking. Ugh! That’s annoying. While I haven’t had the same issues, enough of you mentioned it that I want to put a note here to watch your cakes closely.Batter: you may want to judge the batter in the cake pan by only pouring it 2/3 full and using the rest of the batter for cupcakes, just to make sure you don’t have a mess in your oven.