Preheat your oven to 350°F (175°C). Grease a 9x5 inch loaf pan with butter or cooking spray.
In a large bowl, mash the ripe bananas with a fork until mostly smooth. Stir in the melted butter.
Mix in the sugar, beaten egg, and vanilla extract until well combined.
Sprinkle the baking soda, salt, and cinnamon over the mixture and stir to combine.
Gently fold in the flour until just combined — do not overmix. Fold in the walnuts if using.
Pour the batter into the prepared loaf pan and spread evenly.
Bake for 55–60 minutes, or until a toothpick inserted in the center comes out clean.
Let cool in the pan for 10 minutes, then transfer to a wire rack. Slice and enjoy!