Preheat and Prep: Preheat your oven to 350°F (175°C). Lightly grease a 9x5-inch loaf pan with butter or non-stick spray.
Mash the Bananas: In a large mixing bowl, peel and mash the bananas with a fork until mostly smooth. A few small lumps are fine.
Mix Wet Ingredients: To the mashed bananas, add the melted butter, sugar, egg, and vanilla extract. Whisk until fully combined.
Add Dry Ingredients: In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon (if using). Gradually add to the wet ingredients, stirring just until combined. Do not overmix.
Pour and Bake: Pour the batter into the prepared loaf pan and smooth the top with a spatula. Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Cool and Serve: Let the banana bread cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely before slicing.