Preheat oven to 350 degrees (F). Spray a loaf pan with nonstick cooking spray or grease with butter and set aside.
Add the stick of butter to a large bowl and microwave for 1 minute, or until melted.
Add the bananas to the same bowl and mash with a fork.
Add the vanilla extract and egg to the bowl and use the same fork to mash and stir until no yellow streaks of egg remain.
In a second large bowl, whisk together the flour, sugar, baking soda, salt, and cinnamon.
Add the dry ingredients to the wet ingredients and mix together with a spatula just until combined.
Pour batter into prepared loaf pan and bake for 45 - 55 minutes until a toothpick inserted in the center of the bread comes out clean.
Cool in pan for 10 minutes then turn out onto a wire rack to cool completely.
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