Preheat oven to 350°F (175°C).
Grease a 9×5-inch loaf pan or line it with parchment paper.
Mash bananas in a large bowl until smooth.
Stir in melted butter until well combined.
Add sugar, beaten egg, and vanilla extract. Mix well.
Sprinkle baking soda and salt over mixture and stir gently.
Add flour and mix just until combined (do not overmix).
Pour batter into prepared loaf pan and spread evenly.
Bake 50–60 minutes, until a toothpick comes out clean.
Cool in pan 10 minutes, then transfer to a wire rack to cool completely before slicing.
