Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it lightly.
In a mixing bowl, mash the ripe bananas with a fork until smooth. Stir in the melted butter.
Mix in the baking soda and salt.
Add the sugar, beaten egg, and vanilla extract, and mix until well combined.
Gradually fold in the flour until just incorporated. Be careful not to overmix.
Gently fold in the chopped walnuts.
Divide the batter evenly among the muffin cups, filling each about 2/3 full.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.