In a large mixing bowl, beat the egg until pale and frothy. Add banana purée (2 bananas), buttermilk, coffee, vanilla, sugar and oil and mix until incorporated. Mix in the flour, baking soda, baking powder, cocoa powder, and salt until incorporated. You should have a fairly runny batter. That’s okay. Spoon the batter into a cupcake tin lined with paper liners. Bake for about 20 minutes in an oven preheated to 350F or until a toothpick comes out clean when you poke it into the middle of a cupcake. Let cool on a wire rack. Meanwhile, purée your last banana until it is super super smooth. If there are any chunks, your frosting will look unappetizing. Cream the butter and cream cheese until smooth. Add the banana purée and icing sugar. Feel free to add a little more icing sugar if you find the frosting a little runny. Add vanilla and mix. Transfer to a piping bag, and pipe little clouds of dreamy banana cream cheese frosting on your ever-so-light fluffy cupcakes.