Preheat oven to 350 degrees. Line standard muffin tins with paper liners; set aside. Sift together cocoa powder, flour, sugar, baking soda, baking powder, and salt into a large bowl. Add eggs, warm water, buttermilk, oil, and vanilla, and mix until smooth, about 3 minutes. Scrape down the sides and bottom of bowl to assure batter is well mixed.
Divide batter evenly among muffin cups, filling each 2/3 full. Bake until tops spring back when touched, about 20 minutes, rotating pan once if needed. Transfer to a wire rack; let cool completely.