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. Stir until smooth and well incorporated.
04 - Add the dry ingredients to the wet mixture and fold gently until just combined. Avoid overmixing to prevent a dense crumb.
05 - In a small bowl, stir together 1/4 cup granulated sugar and 1 tablespoon ground cinnamon until uniformly blended.
06 - Pour half the batter into the prepared loaf pan and spread evenly. Sprinkle half of the cinnamon-sugar mixture over the batter in an even layer.
07 - Spread the remaining batter over the swirl layer and smooth the top. Sprinkle the rest of the cinnamon-sugar over the surface. Draw a butter knife through the layers in a gentle figure-eight motion for a marbled effect.
08 - Bake for 50 to 60 minutes, until a toothpick inserted into the center comes out clean.
09 - Let cool in the pan for at least 15 minutes, then turn out onto a wire rack. Cool completely before slicing and serving.