Preheat oven to 350°F. Line muffin tin with paper liners. Set aside.
In the bowl of a stand mixer, cream together butter and sugar until smooth. Add egg and vanilla. Mix until combined. Add in mashed bananas. Mix together. *The mixture may look curdled.
In another bowl, whisk together baking powder, baking soda, and flour. Add half to mixing bowl. Mix on low speed until almost combined. Add milk and mix together. Add remaining flour mixture 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 chunks and chopped walnuts 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.