- 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.