Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.
In a mixing bowl, mash the ripe bananas with a fork until smooth.
Stir in the melted butter until well combined.
Add the baking soda and salt, mixing thoroughly.
Incorporate the sugar, beaten egg, and vanilla extract.
Gradually fold in the all-purpose flour until just combined.
Gently fold in the chopped walnuts or pecans.
Divide the batter evenly among the muffin cups, filling each about two-thirds full.
Bake for 20-25 minutes or until a toothpick inserted comes out clean.
Let the muffins cool in the tin for 5 minutes before transferring them to a wire rack.