Preheat oven to 350 degrees F. Spray one large, or two small, loaf pans with cooking spray.
In a large bowl, mash bananas with a fork until smooth.
Add butter, eggs, sugar, and vanilla to bowl, and mix/mash until fully incorporated. (I usually have small chunks [size of a pea or less] of butter after mixing which is ok!)
Add flour, baking soda, baking powder, salt, and cinnamon to bowl, and mix until flour is barely incorporated, and you can no longer see streaks of flour.
Pour batter into prepared loaf pan(s).
If using walnuts and additional tbsp of sugar, evenly sprinkle both ingredients on top of the batter.
Bake for about 45-50 minutes for one large loaf, or 30 minutes for two small loaves, rotating pan half-way through cooking. The bread will be done when a toothpick inserted into the center of the bread comes out clean.
Let the bread cool in the pan for 5-10 minutes before removing from the pan. The banana bread can be served warm or at room temperature.