Preheat oven to 350°F. Spray a 12-cup muffin pan generously with non-stick cooking spray.

In a large bowl, melt the butter and let cool slightly. Whisk in the sugar, then the egg and vanilla. Stir in the flour and baking soda. Fold in the mashed bananas until just combined.

In a small bowl, melt the butter and then stir in the granulated sugar, dark brown sugar, and ground cinnamon until well combined.

Fill each muffin cup 1/3 full with some batter. Dollop 1 teaspoon of the cinnamon sugar mixture in a few spots on top of the batter in each muffin cup. Use a utensil to gently swirl the cinnamon sugar through the batter in each muffin cup.

Scoop the remaining batter evenly among the muffin cups, filling each muffin cup almost full. Dollop 1 teaspoon of the cinnamon sugar mixture in a few spots on top of the batter in each muffin cup. Use a utensil to swirl the cinnamon sugar through the top half of the muffin batter.

Cut the reserved banana into 12 thin slices and place a slice in the middle of the swirled batter in each muffin cup.

Bake muffins for 20-25 minutes or until golden brown and a toothpick inserted comes out clean. Let the muffins cool in the pan for 5 minutes before removing to a wire rack to cool completely.