Preheat oven to 350°F (175°C) and grease a 9×5-inch loaf pan.
In a large bowl, mix mashed bananas, melted butter, granulated sugar, egg, and vanilla until smooth.
In a separate bowl, whisk flour, baking soda, and salt.
Add to banana mixture and stir just until combined.
In a small bowl, mix cinnamon and brown sugar.
Pour half the batter into the loaf pan. Sprinkle half the cinnamon mix.
Add remaining batter and top with the rest of the cinnamon mix. Swirl gently.
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