Preheat oven to 350 degrees. Prepare muffin pan with paper liners.
Thoroughly mash bananas in a large bowl. Stir in the melted butter. Mix in the sugar, beaten egg and vanilla extract.
Mix in baking soda, salt and cinnamon.
Add the flour and mix just until combined.
Fold in chopped walnuts.
Fill muffin cups and bake for 18-20 minutes.