Preheat oven to 350 °F. Grease and flour 9X5 inch loaf pan.
Mash bananas in large mixing bowl. Add butter, sugar, egg, and vanilla together; mix well. 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 bowl, mix together the ⅓ cup sugar and 1 tbsp cinnamon.
Add ½ 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 in the center of the oven for 50-60 minutes or until a toothpick inserted in the center comes out clean. Cool completely on rack before removing from pan.