Preheat your oven to 350°F (175°C) and prepare a 4x8 inch loaf pan by greasing it with butter or cooking spray.

In a mixing bowl, mash the ripe bananas until smooth, then pour in the melted butter. Mix until fully combined.

Stir in granulated sugar, a beaten egg, and vanilla extract into the banana mixture. Mix until everything is well blended.

Sprinkle baking soda and a pinch of salt over the wet mixture, then gently fold in the all-purpose flour.

In a separate bowl, blend brown sugar and cinnamon to create the swirl mixture.

Pour half of the banana batter into the prepared loaf pan and smooth it out evenly.

Sprinkle three-quarters of the cinnamon-sugar mixture over the first layer of banana batter.

Carefully spoon the remaining banana batter over the cinnamon-sugar layer and smooth the top.

Using a knife, gently swirl through the batter to create a marbled effect.

Sprinkle the remaining cinnamon-sugar mixture over the top of the batter before baking.

Place the loaf pan in the preheated oven and bake for 50–60 minutes.

Once baked, remove the bread from the oven and let it cool in the pan for 15 minutes, then transfer to a wire rack.