Preheat the oven to 350 degrees. Spray a cupcake pan with cooking spray or you can use cupcake liners to line the pan.
Add the bananas to the mixer and turn it on low. Once they are mushy, add the egg and cake mix. Scrap sides and make sure everything is combined. You can just use a bowl if you don't have a stand mixer.
Then pour into the cupcake liners or a muffin pan. Fill them close to the top - you should be able to get 12 muffins out of one batch.
Bake for 15- 17 minutes. Use a toothpick to check for doneness. It should come out clean when you poke a muffin.