In a large mixing bowl combine bananas, butter, sugar and eggs and mix well with a hand blender. In a smaller mixing bowl combine flour, baking soda, baking powder, and salt. Slowing add dry ingredients to banana mixture being sure to combine well. Mix in chopped walnuts if using. Bake for 60 mins at 350 F. Test in centre with skewer to make sure it's done. The key to this recipe is freezing the very ripe bananas then defrosting before use. Pecans or chocolate chips are also also nice substitutions for walnuts.