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.
03 - In a large bowl, combine the mashed bananas, melted butter, granulated sugar, brown sugar, eggs, and vanilla until well blended.
04 - Add the dry ingredients to the wet mixture and stir until just combined. Avoid overmixing to keep the crumb tender.
05 - In a small bowl, mix the 1/4 cup granulated sugar with 1 tablespoon ground cinnamon.
06 - Pour half the batter into the prepared loaf pan. Sprinkle half of the cinnamon-sugar mixture evenly over the batter layer.
07 - Spread the remaining batter over the first layer and smooth the top. Sprinkle the remaining cinnamon-sugar over the surface and swirl lightly with a butter knife for a marbled effect.
08 - Bake for 50 to 60 minutes, or until a toothpick inserted into the center comes out clean.
09 - Let cool in the pan for at least 15 minutes, then transfer to a wire rack to cool completely before slicing.