Pre-heat oven to 350*F. Butter & flour a loaf pan or line bottom with parchment paper & spray sides with cooking spray.
In a stand mixer with the paddle attachment, mix bananas until they are mushy. Add sugar, butter, egg & vanilla until combined.
Sprinkle baking soda & salt on top of banana mixture & then gently mix in flour until just combined. Do not over-mix!
In a small bowl, mix together the ⅓ cup sugar & 1 Tbsp cinnamon.
Add half the batter into the prepared loaf pan. Sprinkle ⅔ of the cinnamon-sugar mixture over the batter. Add the other half of the banana batter & top with the remaining cinnamon-sugar.
Bake for 50-60 minutes. Check doneness with a toothpick. Allow to cool completely before slicing. Keep leftovers in a plastic bag to preserve moistness.