Preheat the oven to 350°F (175°C).

Line a muffin tin with parchment paper or muffin liners.

In a mixing bowl, mash the ripe bananas until smooth.

Add melted butter, egg, and vanilla extract to the mashed bananas and mix well.

In another bowl, whisk together flour, sugar, baking soda, and salt.

Gently fold the dry ingredients into the wet mixture using a rubber spatula.

Stir in the chopped walnuts or pecans.

Spoon the batter into the prepared muffin tins, filling each cup about 2/3 full.

Bake in the preheated oven for 20 minutes or until a toothpick comes out clean.

Let the muffins cool for a few minutes in the pan before transferring them to a wire rack.