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 the melted butter into the mashed bananas.
Mix in the baking soda and salt.
Stir in the sugar, beaten egg, and vanilla extract.
Add the gluten-free flour and mix until just incorporated.
Fold in the chopped walnuts.
Divide the batter evenly among the muffin cups.
Bake in the preheated oven for 20 minutes or until a toothpick inserted into the center comes out clean.
Allow muffins to cool in the tin for a few minutes, then transfer to a wire rack to cool completely.