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

In a large bowl, mash the ripe bananas and add melted butter, granulated sugar, beaten egg, and vanilla. Stir until creamy.

In a separate bowl, whisk together all-purpose flour, baking soda, and salt. Add to the wet mixture, stirring gently.

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

Spoon half of the banana batter into the prepared loaf pan, add half the cinnamon-sugar mixture, and repeat with remaining batter and swirl.

Gently swirl the batter with a knife to create a marbled effect.

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

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