1/2 cup (113 grams) unsalted butter, room temperature
1 cup (120 grams) confectioners (powdered or icing) sugar, sifted
1 teaspoon pure vanilla extract
Banana Chocolate Cupcakes: Preheat oven to 350 degrees F (180 degrees C) and place rack in the center of the oven. Line 12 regular-sized muffin cups with paper liners or spray each cup with a non stick vegetable spray.
In a large bowl whisk together the sugar, flour, cocoa powder, baking powder, baking soda, and salt.
In another large bowl, whisk together the egg, mashed banana, water, milk, oil, and vanilla extract. Add the wet ingredients to the dry ingredients and stir, or whisk, until combined. (The batter is quite thin.) Pour or scoop the batter into the muffin cups, about 3/4 full, and bake for about 20 minutes or until a toothpick inserted in the center of a cupcake comes out clean.
Remove from oven and let cool on a wire rack. Frost with the Chocolate Frosting.
Chocolate Frosting: Melt the chocolate in a heatproof bowl placed over a saucepan of simmering water. Remove from heat and let cool to room temperature.
In the bowl of your electric mixer, or with a hand mixer, beat the butter until smooth and creamy (about 1 minute). Add the sugar and beat until it is light and fluffy (about 2 minutes). Beat in the vanilla extract. Add the chocolate and beat on low speed until incorporated. Increase the speed to medium-high and beat until frosting is smooth and glossy (about 2 -3 minutes).