Preheat oven to 350°F (175°C)
Grease and line a 9×5-inch loaf pan
Mash 3 very ripe bananas in a bowl
Whisk in 1/3 cup melted butter (or neutral oil)
Whisk in 3/4 cup sugar (or brown sugar)
Whisk in 1 large egg
Stir in 1 tsp vanilla extract
Mix in 1 tsp baking soda
Add 1/2 tsp salt
Stir in 1 1/2 cups all-purpose flour until just combined
Optional: fold in 1/2 cup chopped walnuts or chocolate chips
Pour batter into the prepared pan
Bake 50–65 minutes, until a toothpick comes out mostly clean
Cool in the pan 10 minutes
Remove to a wire rack to cool completely before slicing