01 - Preheat oven to 350°F. Grease and flour a 9x5-inch loaf pan.
02 - In a medium bowl, whisk together the flour, baking soda, baking powder, salt, and 1 1/2 teaspoons cinnamon until evenly distributed.
03 - In a large bowl, combine the mashed bananas, melted butter, granulated sugar, brown sugar, eggs, and vanilla extract. Mix until fully incorporated and smooth.
04 - Add the dry ingredient mixture to the wet ingredients and fold until just combined. Avoid overmixing to prevent a dense texture.
05 - In a small bowl, stir together 1/4 cup granulated sugar and 1 tablespoon ground cinnamon until evenly blended.
06 - Pour half the batter into the prepared loaf pan and spread evenly. Sprinkle half of the cinnamon-sugar mixture over the batter layer.
07 - Pour the remaining batter over the swirl layer and smooth the top. Sprinkle the remaining cinnamon-sugar mixture evenly across the surface. Draw a butter knife through the layers in a gentle figure-eight motion to create a marbled effect.
08 - Bake for 50 to 60 minutes, or until a toothpick inserted into the center emerges clean.
09 - Let the loaf rest in the pan for at least 15 minutes, then turn out onto a wire rack to cool completely before slicing.