YIELD: MAKES TWO 9-INCH CAKE LAYERS AND I’VE HAD GREAT SUCCESS MAKING IT IN A 9X13-INCH PAN (I SUSPECT CUPCAKES WOULD WORK OUT GREAT, ALSO)
This cake is best made in cake pans that have sides at least 3-inches high so it doesn't overflow while baking.

1 ¼ cups unsweetened cocoa powder
2 ½ cups all purpose flour
2 ½ cups sugar
2 ½ teaspoons baking soda
1 ¼ teaspoons baking powder
1 ¼ teaspoons salt
2 large eggs plus 1 large egg yolk
1 ¼ cups warm water
1 ¼ cups buttermilk
½ cup plus 2 tablespoons vegetable oil
1 ½ teaspoons pure vanilla extract

Preheat the oven to 350 degrees, making sure the baking rack is in the middle of the oven. Prepare your 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.

Recipe Source: adapted slightly from Martha Stewart Living The New Classics by Mel's Kitchen Cafe