Preheat the oven to 350˚F.
Put the 10 paper baking cups on the muffin pan. Grease the top of the muffin pan with the melted butter to prevent sticking during baking.
In a large mixing bowl, sift together the flour, sugar, baking powder, baking soda, and salt. In an electric mixer fitted with a paddle, add the flour mixture and 4 ounces melted butter. Blend to combine.
In a medium bowl, combine the egg, milk, vanilla extract, and mashed bananas (save the diced banana for now). Add the wet ingredients to the flour mixture. Blend everything together, but make sure not to overmix.
Fold in the diced bananas into the batter. Scoop the batter into the baking cups. Fill each cup to the top.
Bake for 25 minutes. Make sure the tops are brown and a toothpick comes out clean. Serve and eat.
For storage, cool the muffins completely. Wrap each muffin in a siren wrap, and put them in the freezer. Reheat the muffin for 15 minutes in a 350˚F oven.