Preheat the oven to 425˚F. Line a 12-cup muffin tin with parchment liners, or spray the cups well with nonstick cooking spray.
In the bowl of a stand mixer fitted with the paddle attachment, add mashed bananas, melted butter, sugar, egg, vanilla, and milk. Beat until just combined.
Add in flour, baking soda, baking powder, and salt. Beat until just combined.
Fold in walnuts and chocolate chips.
Spoon the thick batter into liners, filling the cups all the way to the top.
Bake for 5 minutes, then reduce heat to 350˚F. Bake for an additional 15 to 18 minutes, until aa toothpick inserted in the center comes out clean and the tops are golden brown.
Cool for 5 minutes in the pan, then remove to cooling rack to cool the rest of the way.
Store cooled muffins in an airtight container at room temperature for up to 3 days.