In a small bowl, combine the 1/4 cup light brown sugar and 1 tsp ground cinnamon. Add the 2 tablespoons of melted butter and mix well until a thick paste forms. Set aside.

Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan, or line with parchment paper leaving an overhang on the long sides for easy removal.

In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set aside.

In a large bowl, mash the very ripe bananas with a fork until mostly smooth but with a few small lumps remaining. Stir in the granulated sugar and 1/4 cup light brown sugar until well combined.

Add the 1/2 cup melted butter, large egg, and vanilla extract to the banana mixture. Whisk until everything is fully incorporated and smooth.

Gradually add the dry ingredients to the wet ingredients, mixing with a spatula until just combined. Be careful not to overmix; a few lumps are fine.

Pour about half of the banana bread batter into the prepared loaf pan and spread evenly.

Dollop half of the cinnamon swirl mixture over the batter in small spoonfuls. Spread gently with the back of a spoon if needed, but don't mix it completely into the batter.

Carefully spoon the remaining banana bread batter over the cinnamon swirl layer, spreading it to cover.

Dollop the remaining cinnamon swirl mixture on top of the second batter layer. Use a butter knife or skewer to gently swirl the cinnamon mixture into the top layer of batter, creating a marbled effect.

Bake for 55-65 minutes, or until a wooden skewer or toothpick inserted into the center of the bread comes out clean. If the top starts to brown too quickly, you can loosely tent it with aluminum foil.

Remove the loaf pan from the oven and let it cool in the pan for 10-15 minutes. Then, carefully lift the bread out of the pan using the parchment paper overhang (if used) or invert onto a wire rack to cool completely before slicing and serving.