Preheat oven to 350℉ (177℃). Lightly grease and flour an 8.5x4.5-inch loaf pan or spray with baking spray.
In a large bowl, combine the mashed bananas with the melted butter, then add the sugar, egg, and vanilla. Stir well with a large wooden spoon.
3-4 ripe bananas, 1/3 cup salted butter, 3/4 cup (150g) granulated sugar, 1 large egg, 1 teaspoon pure vanilla extract
Sprinkle the baking soda, baking powder, cinnamon, and salt over the banana mixture. Stir to combine.
1 teaspoon baking soda, 1 teaspoon baking powder, 1/4 teaspoon ground cinnamon, 1/8 teaspoon salt
Add flour and walnuts or chocolate chips (if desired), and stir just until no streaks of flour remain. Pour the batter into the prepared loaf pan.
1 1/2 cups (212g) all-purpose flour, 1 cup chopped walnuts
Bake for 50-60 minutes, until a knife inserted in the center of the loaf comes out clean. Cool in the pan for 10 minutes, then turn out onto a wire rack and cool completely. Slice into 10 thick slices and serve.