This crowd-pleaser is a no-fail, can't-miss recipe that every baker should have in their back pocket.
Preheat oven to 350 degrees. Coat a large 12-cup muffin tin with cooking spray. Sift cocoa, flour, sugar, baking soda, baking powder, and salt into large bowl. Stir in eggs, water, buttermilk, oil, and vanilla until smooth, about 3 minutes.
Spoon batter into cups. Bake until cake tester comes out clean, 20 to 25 minutes. Let cool in tin 10 minutes. Unmold; let cool completely on wire racks.
To make Tea Party Cake, double the recipe; spread 3 1/2 cups batter into another; bake at 350 degrees for 30 minutes for the thinner cake and 50 minutes for the thicker.