Preheat oven to 350°F (180°C)
Line a 9×5-inch loaf pan with parchment paper
Mash bananas with a fork until mostly smooth
Mix in melted butter, sugar, egg, and vanilla
Whisk flour, baking soda, and salt
Gradually stir dry ingredients into wet mixture
Scrape batter into prepared pan
Bake 50-60 minutes until golden and a toothpick inserted comes out clean
Cool in pan 10 minutes, then transfer to wire rack to cool completely
Slice and serve