Spray your loaf pan with a nonstick spray and set aside.
In a large bowl whisk together the flour, sugar, baking soda, salt, and cinnamon and set aside.
In a second large bowl with the melted butter, add bananas and mash with a fork until you have well mashed bananas. Add in the vanilla extract and egg. Combine the dry ingredients with the wet ingredients. Mix together until just combined.
Pour the batter into the prepared loaf pan and sprinkle chopped walnuts on top (optional). Bake for 45-55 minutes or until a toothpick inserted into the center comes out clean.