Preheat oven to 350. Butter and flour a 9x5 loaf pan.
Mix bananas, butter, sugar, egg, and vanilla together. sprinkle baking soda and salt around on top of the banana mixture. Then gently stir in flour. Be careful not to over-mix!
In a small dish, mix together the 1/3 cup sugar and 1 Tbs cinnamon.
Add 1/2 of the batter to the loaf pan and then sprinkle half, or a little more than half of the cinnamon-sugar mixture all over the batter in the pan. Add the rest of the batter, and then sprinkle the leftover cinnamon-sugar on top.
Bake for 50-60 minutes, but remember, if you’re using a different-sized loaf pan, be careful and keep a close eyeball on it!