Preheat the oven to 350°F (175°C).
Grease or line a 9x5-inch loaf pan.
Mash the bananas until mostly smooth.
Whisk together the melted butter, sugar, egg, and vanilla.
Stir in the mashed bananas.
In another bowl, combine the flour, baking soda, salt, and cinnamon.
Fold the dry ingredients into the wet mixture until just combined.
Stir in walnuts or chocolate chips if using.
Pour the batter into the prepared loaf pan.
Bake for 50–55 minutes or until a toothpick inserted into the center comes out clean.
Cool in the pan for 10 minutes.
Transfer to a wire rack and cool completely before slicing.
