Preheat your oven to 350°F.
Line a muffin tin with paper liners or grease it lightly.
In a medium bowl, mix together the flour, baking powder, baking soda, and salt.
Set aside for later.
In a separate bowl, combine sugar and melted butter.
Add the mashed bananas, egg, and vanilla extract.
Mix until smooth.
Slowly add the dry mixture to the wet mixture.
Stir until just blended.
Fold in chopped walnuts gently.
Divide the batter evenly among the muffin cups.
Fill each about 3/4 full.
Bake for 20-25 minutes or until golden brown.
Let them cool before serving.