Preheat oven to 350°F. Prepare an 8x4-inch loaf pan by spraying with nonstick spray.
Melt butter and set aside to cool.
In a large bowl, mash bananas with a fork. (You can use up to 5 regular sized bananas or 6 small bananas in this recipe.)
Add butter, sugar, egg, and vanilla. Mix well.
Add in flour, baking soda, baking powder, and salt and mix until ingredients are JUST incorporated. (Be careful not to overmix.)
Pour the batter into the prepared loaf pan.
Bake for 45 minutes. (Cover with aluminum foil if browning too quickly.)
Bake for an additional 15-20 minutes, or until a toothpick comes out with crumbs, but not wet batter.