Preheat oven to 400 degrees F. Prepare a muffin pan with 6 muffin liners. In a large bowl, mash bananas until only small chunks remain. Add in melted (and cooled) butter and brown sugar and stir until well combined. Stir in beaten eggs and vanilla and stir until combined. In a separate bowl combine flour, baking soda, baking powder and salt, stir to combine. Combine wet and dry mixture until just combined. Do not over-mix. If desired, add in optional nuts and stir to combine. Divide batter between 6 muffin cups. Place in oven and immediately reduce the oven temperature to 350 degrees F. Bake for approximately 20 minutes or until a toothpick inserted into the center of a muffin comes out clean, and tops are turning golden brown.