Preheat your oven to 350°F (175°C) and grease the loaf pan.
In a mixing bowl, combine mashed bananas, melted butter, baking soda, salt, sugar, egg, and vanilla extract. Mix well.
Stir in flour until just combined.
In a small bowl, mix together ⅓ cup of sugar and cinnamon for the swirl.
Pour half of the banana batter into the prepared loaf pan. Sprinkle half of the cinnamon sugar mixture over the batter. Add the remaining batter and top with the rest of the cinnamon sugar mixture.
Bake for 60 minutes or until a toothpick inserted into the center comes out clean.
Allow the bread to cool in the pan for a few minutes before transferring to a wire rack to cool completely.
Notes
This banana bread can be stored in an airtight container for several days. You can also add walnuts or chocolate chips for extra flavor.