Preheat the oven to 350°F (175°C). Grease a 9×5-inch loaf pan or line it with parchment paper.

In a large bowl, mash the bananas with a fork until smooth.

Pour in the melted butter and stir until combined.

Add the sugar, beaten egg, and vanilla. Stir until evenly mixed.

Sprinkle the baking soda and salt over the mixture and stir gently.

Add the flour and fold just until no flour streaks remain. Stop as soon as the batter is uniform—overmixing makes the loaf tough.

Pour the batter into the prepared pan and smooth the top.

Bake for 50 to 60 minutes. Check at 50 minutes by inserting a toothpick into the center; it should come out clean or with a few moist crumbs.

Let the bread cool in the pan for 10 minutes.

Transfer the loaf to a wire rack to cool completely before slicing to avoid a gummy center.