Preheat oven to 350°F (175°C). Grease a muffin tin or line with paper liners.

In a large bowl, mash the bananas until mostly smooth. Stir in the melted butter until combined.

Add the sugar, beaten egg, and vanilla. Mix until the mixture is uniform.

Sprinkle the baking soda and salt over the batter and fold them in gently.

Add the flour gradually, stirring just until no streaks of flour remain. Avoid overmixing.

Divide the batter among the prepared muffin cups, filling each about two thirds full.

Bake for 18 to 20 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.

Let the muffins cool in the tin for a few minutes, then transfer to a wire rack to cool completely.