Spray a 12 well muffin pan with non-stick spray, or use paper liners.
Whisk together the flour, baking powder, baking soda, and salt in a small bowl and set aside.
Combine the mashed bananas, white and brown sugars, egg, vanilla and melted butter in a large bowl.
Mix in the flour mixture and combine until smooth., then stir in the walnuts.
Bake in the preheated oven.
Bake the muffins for 20-25 minutes. They muffisn are done when a toothpick inserted comes out clean and the muffin tops spring back when lightly touched.
Notes
If you are making mini muffins bake them for 13-15 minutes.