In a small bowl, whisk together 2 tablespoons melted butter, 1/3 cup packed light brown sugar, and 1 teaspoon ground cinnamon until smooth.

Preheat oven to 350°F (177°C). Grease a 9×5-inch loaf pan and line with parchment, leaving an overhang.

In a large bowl, mash 3 ripe bananas until mostly smooth. Stir in 1/2 cup melted butter, 1/2 cup granulated sugar, 1/4 cup packed light brown sugar, 2 lightly beaten eggs, and 1 teaspoon vanilla extract until streaks of butter disappear.

In a separate bowl, whisk 1 3/4 cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon ground cinnamon, and 1/2 teaspoon salt.

Gently fold the dry ingredients into the wet until just combined; the batter should be thick and slightly lumpy.

Spoon half the batter into the prepared pan and smooth the top. Bake for 15 minutes, until the edges look set and the center is matte.

Remove pan from oven. Dollop the cinnamon sugar paste down the center and use a knife to swirl, keeping the swirl to the middle.

Cover the swirl with the remaining batter, spreading to seal. Return to the oven and bake for 30–35 minutes, until a toothpick inserted into the center comes out with moist crumbs.

Cool the loaf in the pan on a wire rack for 10 minutes. Use the overhanging parchment to lift it out, then let cool completely before slicing.