Preheat the oven to 350°F (175°C) and grease a 9x5-inch loaf pan.

Combine the mashed bananas, melted butter, granulated sugar, eggs, and vanilla extract in a large bowl. Mix until smooth.

Whisk together the all-purpose flour, baking soda, and salt in a separate bowl. Slowly add this dry mixture to the banana blend, stirring gently.

Mix the brown sugar and ground cinnamon in a small bowl. Pour half of the banana batter into the loaf pan, sprinkle half the cinnamon mixture, add the remaining batter, and cover with the remaining cinnamon blend. Swirl gently.

Bake for 50-55 minutes and check for doneness with a toothpick. Let cool in the pan for 10 minutes before transferring to a wire rack.