Preheat the oven to 350°F (175°C) and prepare a 4x8-inch loaf pan with non-stick spray or butter.

In a large bowl, mash the ripe bananas until smooth and blend in the melted butter.

Stir in sugar, beaten egg, and vanilla extract until well combined.

Sprinkle baking soda and salt over the wet mixture, then fold in the all-purpose flour until just combined.

In a separate bowl, mix together the cinnamon and sugar for the swirl.

Pour half of the banana batter into the prepared loaf pan.

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

Spoon the remaining batter over the cinnamon-sugar layer.

Gently swirl through the batter using a butter knife or skewer to create a marbled effect.

Sprinkle the leftover cinnamon-sugar mixture over the top of the batter.

Bake in the preheated oven for 50-60 minutes or until a toothpick comes out clean.

Allow the bread to cool in the pan for 15 minutes before transferring to a wire rack to cool completely.