Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.
In a large bowl, whisk together melted butter and sugar.
Add the eggs and vanilla extract, mixing until smooth.
Stir in the mashed bananas.
In a separate bowl, combine flour, baking soda, salt, and cinnamon.
Gently fold the dry ingredients into the wet mixture until just combined.
Fold in the chopped nuts.
Divide the batter evenly among the muffin cups, filling each about three-quarters full.
Bake for 18–22 minutes, or until a toothpick inserted in the center comes out clean.
Let cool for 5 minutes in the pan, then transfer to a wire rack to cool completely.