Preheat oven to 350˚F. Grease muffin tin or line with parchment cups.
Melt butter and set aside.
In a large bowl combine the bananas, apple sauce, sugar, eggs, flour, baking soda, baking powder, walnuts, chocolate chips and vanilla. Stir just to mix. It will be thick.
Pour melted butter over and stir to combine. Do not over mix.
Using a ¼ cup measuring cup, scoop into muffin pan. Ensure muffin mix is evenly distributed for even baking.
Bake for 20 minutes or until a toothpick inserted comes out clean.