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. Beat until smooth and well incorporated.
04 - Add the dry ingredients to the wet mixture and fold until just combined. Take care not to overmix to keep the crumb tender.
05 - In a small bowl, stir together 1/4 cup granulated sugar and 1 tablespoon ground cinnamon until uniform.
06 - Pour half the batter into the prepared loaf pan and spread evenly. Sprinkle half the cinnamon-sugar mixture over the surface. Top with the remaining batter and smooth, then finish with the rest of the cinnamon-sugar. Draw a butter knife through the layers in a gentle swirling motion for a marbled effect.
07 - Bake for 50 to 60 minutes, until a toothpick inserted into the center comes out clean.
08 - Let cool in the pan for at least 15 minutes, then turn out onto a wire rack to cool completely before slicing.