Preheat your oven to 350 F.
In a medium bowl, combine the mashed bananas with melted butter. Add the egg, sugar, and vanilla extract; mix until smooth.
Fold in the flour, baking soda (or baking powder), and salt until just combined—avoid over-mixing.
Pour the batter into a greased 8×4 inch loaf pan.
Bake for about one hour or until a toothpick inserted in the center comes out clean.
Cool in the pan for a few minutes before transferring to a wire rack to cool completely.
