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.
Mix in the baking soda and salt.
Add the sugar, beaten egg, and vanilla extract; mix until smooth.
Gradually add the flour, mixing just until incorporated.
Fold in the chopped walnuts gently.
Spoon the batter into the muffin liners, filling each about 2/3 full.
Bake in the preheated oven for 20-25 minutes or until a toothpick comes out clean.
Allow to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.