Preheat oven to 350 degrees. Spray a loaf pan with non-stick 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 the batter into prepared loaf pan and bake for 45-55 minutes until a toothpick inserted in the center of the bread comes out clean.