Line your muffin pan with 12 paper liners or spray a 9×5″ loaf pan with cooking spray
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 tbsp cinnamon
Fill the paper liners halfway with batter then evenly distribute 1/2 the cinnamon-sugar mixture over the batter (or 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 to the paper liners and then sprinkle the leftover cinnamon-sugar on top (or sprinkle the rest of the cinnamon-sugar all over the loaf)
Bake for 20-25 minutes (50-60 minutes for a 9×5″ loaf)