Preheat your oven to 350°F (175°C) and grease a 9x5-inch loaf pan with butter or non-stick spray.

Mash the 3 ripe bananas until smooth, then mix in the melted butter, granulated sugar, beaten egg, and vanilla extract until creamy.

In a separate bowl, whisk together flour, baking soda, and salt. Gradually fold these dry ingredients into the wet banana mixture.

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

Pour half of the banana batter into the loaf pan, sprinkle half of the cinnamon mixture, then layer the remaining batter and swirl mixture.

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

Bake for 60 to 70 minutes, or until a toothpick comes out clean. Tent with foil if browning too quickly.

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