Preheat oven to 350°F
In a bowl, cream together the melted butter and sugar, then blend in the beaten egg, mashed bananas and vanilla. Mix well.
In a separate bowl, thoroughly blend together the flour, baking soda, baking powder and salt.
Combine both mixtures.
Add walnuts, then fold into the batter.
Coat muffin pans with cooking spray, then fill each cavity evenly with the batter (about 2/3 of cavity).
Bake at 350°F for 20-25 minutes until golden brown and when inserted toothpick comes out clean.
Let cool for 2-3 minutes then serve warm.