Preheat oven to 350 degrees F.

In the bowl of an electric mixer, beat butter and sugar until fluffy – about 3-4 minutes. Add egg and vanilla, and mix until combined, scraping down the bowl a few times. With the mixer on medium speed, add in flour, salt and cocoa powder, mixing until just combined. Line a cupcake tin with liners, then spoon about 1/2-3/4 of a tablespoon of brownie batter in each cup. Press down with the back of a spoon coated in non-stick spray.

Combine dry flour, salt, baking soda, cinnamon and nutmeg and set aside. In a bowl, whisk egg and add brown sugar, mixing until smooth. Stir in vanilla extract. Add sour cream and melted butter and mix. Stir in dry ingredients, then add mashed bananas and mix until batter comes together. The batter will be thicker than normal cupcakes. Using an ice cream scoop or 1/4 cup measure, scoop batter in liners over top of brownie batter. Bake for 15-18 minutes, then let cool completely before frosting.

Beat butter and cream cheese in the bowl of your electric mixer until completely combined and creamy, scraping down the sides a few times. Add in brown sugar and powdered sugar, beating on low speed and gradually increasing to medium speed. Add in vanilla extract. Beat frosting for 4-5 minutes on medium-high speed until really creamy and combined. This frosting is not very thick or study, so make sure to either frost immediately before eating, or keep in the refrigerator for up until 30 minutes before serving.