1. Preheat oven to 350 degrees. Line three 12-cup mini muffin trays with paper liners. In a large mixing bowl, combine flour, sugar, baking powder, baking soda, and salt; set aside.

2. With an electric mixer, combine cocoa and 3 tablespoons hot water until a thick paste forms. Add butter, buttermilk, egg, and egg white; beat until combined. Whisk in flour mixture until smooth.

3. Transfer batter to prepared tins. Each cup should be about 2/3 full. Bake until a toothpick inserted into the center of a cupcake comes out clean, 12 to 15 minutes. Transfer cupcakes to a wire rack to cool completely.

4. Use an offset spatula or butter knife to spread icing over tops. Decorate as desired.