Preheat your oven to 350°F (175°C). Grease a 4x8-inch loaf pan.

In a mixing bowl, mash the bananas. Stir in the melted butter, sugar, beaten egg, and vanilla extract.

Add the baking soda, salt, and flour. Mix until just combined.

In a small bowl, mix the 1/3 cup sugar and cinnamon for the swirl.

Pour half of the banana bread batter into the prepared pan. Sprinkle half of the cinnamon sugar mixture over the batter. Add the remaining batter, then top with the rest of the cinnamon sugar.

Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.

Let the bread cool in the pan for 10 minutes before transferring to a wire rack.