Preheat your oven to 350°F (175°C) and grease a 9x5-inch loaf pan.

Mash three ripe bananas until smooth and combine with melted butter, sugar, egg, and vanilla extract.

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

Gradually add the dry mixture to the wet ingredients, folding gently.

Mix brown sugar and cinnamon in a small bowl.

Pour half of the banana batter into the loaf pan, add half of the cinnamon mixture, then top with remaining batter and cinnamon mixture.

Gently swirl the cinnamon mixture into the batter with a knife.

Bake for 60 to 70 minutes, checking for doneness with a toothpick.

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