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

In a large bowl, combine mashed ripe bananas and melted butter. Stir in sugar, beaten egg, and vanilla extract until well mixed.

In another bowl, whisk together flour, baking soda, and salt. Gradually add to the banana mixture, stirring gently.

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

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

Use a knife to gently swirl the batter to create a marbled effect.

Bake in the oven for 60-70 minutes, checking for doneness with a toothpick.

Allow the bread to cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely.