Preheat the oven to 350°F (175°C). Grease a 9×5-inch loaf pan or line it with parchment paper.
Mash the ripe bananas in a large mixing bowl.
Add melted butter, sugar, egg, and vanilla extract. Stir until well combined.
In a separate bowl, whisk together flour, baking soda, salt, and cinnamon.
Gradually add the dry ingredients to the banana mixture. Stir just until combined. Do not overmix.
Pour the batter into the prepared loaf pan and smooth the top.
Bake for 50–60 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Cool in the pan for 10–15 minutes, then transfer to a wire rack to cool further before slicing.