Preheat the oven to 350 F. Line 2 standard 12 cup muffin pans with paper liners.Into the bowl of an electric mixer, sift together flour, cocoa, sugar, baking soda, baking powder and salt. Attact bowl to mixer fitted with the paddle attachment; add the eggs and yolk, the milk, oil, vanilla and warm water. Beat on low speed until smooth and combined, about 3 minutes; scrape down the sides of the bowl as needed.
Divided batter evently among the muffin cups, filling each about 2/3 full. Bake, rotating pans halfway through, until a cake tester inserted in the center of a cupcake come out clean, 20-25 minutes. Transfer pans to a wire rack to cool slightly. Invert cupcakes onto the rack; then reinvert and let them cool completely, top sides up. Frost cupcakes with buttercream, swirling to cover and garnish with chocolate curls.