Melt the butter. Set aside. Combine the flour, baking soda, baking powder and salt in a small mixing bowl. Set aside. With your electric mixer, beat the bananas until they are mashed up. Mix in the eggs, sugar and melted butter. Beat on medium speed until the mixture is smooth.
Add in the vanilla and the nuts (if desired). Cut a piece of parchment paper that is the same width as the long side of your loaf pan but is long enough to cover the bottom, the sides and fold over the top edge of the pan. Insert the parchment paper into your loaf pan so that it covers the bottom and the sides, and is sticking up over the top on both sides.