Preheat oven to 350 degrees Fahrenheit. Butter and flour an 8x4-inch bread pan or use parchment paper on the bottom and cooking spray on the sides.
In a large bowl, mash bananas thoroughly with a fork, until smooth. Add melted butter, sugar, egg, and vanilla. Mix well.
Add the flour, baking soda and salt. Stir the dry ingredients slightly before folding them into the wet batter. Mix until flour barely disappears.
Pour batter into the prepared bread pan. Bake for 50-60 minutes, or until a toothpick comes out clean from the center.
Allow bread to cool in pan for 5 minutes before inverting; then cool completely on a cooling rack. Slice with a serrated knife and serve. Wrap leftovers in plastic wrap to keep fresh up to 1 week.