Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it lightly.
In a mixing bowl, combine the mashed bananas and melted butter.
Stir in the baking soda and salt.
Mix in the sugar, beaten egg, and vanilla extract until well combined.
Gradually add the flour, stirring until just incorporated. Do not overmix; a few lumps are okay.
Spoon the batter into the prepared muffin tin, filling each cup about two-thirds full.
Bake for 18 to 20 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.