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

In a large bowl, mix the mashed bananas, melted butter, granulated sugar, beaten egg, and vanilla extract until well combined.

In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, stirring just until incorporated.

In a small bowl, combine the ground cinnamon and brown sugar to create the cinnamon swirl mixture.

Pour half of the banana bread batter into the loaf pan. Sprinkle half of the cinnamon mixture evenly over the batter. Pour the remaining batter on top, then sprinkle the remaining cinnamon mixture.

Using a knife or skewer, gently swirl the cinnamon mixture through the batter to create a marbled effect.

Bake for 60-70 minutes, or until a toothpick inserted in the center comes out clean. If the bread starts to brown too quickly, tent with foil.

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