01 - Preheat oven to 350°F (175°C) and grease a standard loaf pan.
02 - In a large bowl, whisk together mashed bananas, melted butter, granulated sugar, egg, and vanilla extract until well mixed.
03 - Add all-purpose flour, baking soda, and salt to the wet mixture and stir gently until just combined. Do not overmix.
04 - In a separate small bowl, combine brown sugar and ground cinnamon.
05 - Pour half of the batter into the prepared pan. Sprinkle half of the cinnamon sugar mixture evenly over the batter. Add remaining batter, then top with the rest of the cinnamon sugar. Swirl gently using a knife.
06 - Bake for 50 to 60 minutes, or until a toothpick inserted in the center comes out clean. Allow bread to cool in the pan before slicing.