Preheat oven to 350. Mix together mashed up bananas, melted butter and milk. Lightly beat egg. Whisk egg, vanilla and sugar into other ingredients. Add dry ingredients and stir until everything is just mixed together. Do not over-mix. Spray a muffin pan and fill each muffin cup 4/5 full (nearly full). Bake for 20 minutes. Makes 12 standard size muffins.