Preheat the oven to 375F and line a muffin tin.

Mash the bananas in a large bowl until mostly smooth.

Stir in the melted butter, sugar, eggs, and vanilla until combined.

Add the flour and baking soda and stir gently until just mixed, then fold in the walnuts.

Divide among the cups and bake 18 to 20 minutes until a toothpick comes out clean.

Cool in the pan for 5 minutes before transferring to a rack.