Yield: one dozen
Preheat oven to 350 degres F. Line muffin tin with cupcake liners. Set aside.
In the bowl of a stand mixer, cream together butter and sugar until smooth. Add in mashed bananas, egg, vanilla, and milk. Mix until combined.
In another bowl, whisk together baking powder, baking soda, and flour. Add to mixing bowl. Mix on medium speed until there are no longer any dry streaks of flour. Scrape down bowl as needed to ensure thorough mixing.
Fold in chocolate chips, walnuts, and peanut butter chips until distributed. Divide batter among prepared muffin tin.
Bake for 20-23 minutes, until a toothpick inserted in the center of the muffin comes out clean. Allow muffins to cool in pan for 3-5 minutes before unmolding. Allow to cool on wire rack.
Adapted From King Arthur Flour