Preheat oven to 350 degrees. Line standard muffin tins with paper liners. Whisk together flour, cocoa powder, sugar, baking soda, baking powder, and salt. Add eggs, buttermilk, oil, water, and peppermint extract, and beat with a mixer on low speed until smooth.
Divide batter among muffin cups, filling each 2/3 full. Bake, rotating tins halfway through, until a tester inserted in the center comes out clean, 20 to 25 minutes. Let cupcakes cool in tins on wire racks for 10 minutes. Transfer cupcakes to racks, and let cool. Undecorated cupcakes will keep, covered, for 1 day, or frozen for up to 2 months.
Spoon buttercream into a pastry bag fitted with a large, plain tip. Pipe small mounds on tops of cupcakes. Flatten mounds with an offset spatula, then top with chocolate-mint leaves.