Preheat your oven to 350°F (175°C) and grease a 9x5-inch loaf pan.
In a mixing bowl, combine the mashed bananas and melted butter.
Stir in the baking soda and salt.
Add the sugar, beaten egg, and vanilla extract, mixing until well combined.
Gradually add the flour, mixing until just incorporated.
In a small bowl, mix the ground cinnamon and brown sugar together.
Pour half of the banana bread batter into the prepared loaf pan.
Sprinkle half of the cinnamon-sugar mixture over the batter.
Pour the remaining batter on top and finish with the rest of the cinnamon-sugar mixture.
Use a knife to swirl the mixture gently into the batter.
Bake for 60-65 minutes, or until a toothpick inserted into the center comes out clean.
Allow the bread to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.