Preheat oven to 350 degrees. Line mini muffin tins with baking cups. Sift flour, sugar, cocoa, baking soda, baking powder, and 1 teaspoon salt into a large bowl. Mix in eggs, warm water, buttermilk, oil, and vanilla, scraping down sides of bowl, until smooth, about 3 minutes.
Working in batches, divide batter evenly among cups, filling each two-thirds full. Bake, rotating tins halfway through, until tops spring back when pressed with a finger, about 20 minutes. Let cupcakes cool completely on a wire rack.
Using a pastry bag fitted with a fine star tip, pipe buttercream onto cupcakes to decorate.
Unfrosted cupcakes can be stored at room temperature for up to 3 days.