Preheat your oven to 350°F (175°C) and grease a 9×5-inch loaf pan.

In a mixing bowl, combine mashed bananas, melted butter, sugar, egg, and vanilla extract until creamy.

In a separate bowl, whisk together flour, baking soda, and salt.

Fold the dry ingredients into the wet mixture until just combined.

Combine cinnamon and brown sugar in a small bowl for the swirl.

Layer half of the batter in the pan, sprinkle with half of the cinnamon-sugar mixture, and then add the rest of the batter and remaining cinnamon mixture.

Swirl with a knife gently to create marbled effect.

Bake for 60-70 minutes or until a toothpick comes out clean.

Cool in the pan for 10 minutes before transferring to a wire rack to cool completely.