Preheat your oven to 350°F (175°C). Grease a 9x5 inch loaf pan or line with parchment paper.
In a large mixing bowl, mash the bananas with a fork, leaving some small chunks for texture. Stir in the melted butter until combined.
Add the brown sugar, beaten egg, and vanilla extract to the banana mixture. Mix until well combined.
Sprinkle the baking soda, salt, and cinnamon (if using) over the mixture and stir to combine.
Add the flour and gently fold it in until just combined. Don't overmix – a few lumps are okay. Fold in walnuts if using.
Pour the batter into the prepared loaf pan and smooth the top gently.
Bake for 55-65 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs.
Cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely before slicing.
Notes
Storage: Wrap tightly and store at room temperature for up to 4 days, or freeze for up to 3 months. For best results, use very ripe bananas with brown spots. The bread tastes even better the next day as the flavors develop.