01 - Preheat oven to 350°F. Grease and flour a 9x5-inch loaf pan.
02 - Whisk together the flour, baking soda, baking powder, salt, and 1 1/2 teaspoons cinnamon in a medium bowl.
03 - Combine the mashed bananas, melted butter, granulated sugar, brown sugar, eggs, and vanilla in a large bowl until well blended.
04 - Fold the dry ingredients into the wet mixture and stir until just combined. Avoid overmixing to maintain a tender crumb.
05 - Mix 1/4 cup granulated sugar with 1 tablespoon ground cinnamon in a small bowl.
06 - Spread half the batter into the prepared pan. Sprinkle half the cinnamon-sugar mixture evenly over the surface.
07 - Add the remaining batter and smooth the top. Scatter the rest of the cinnamon-sugar over the batter and draw a butter knife through the layers for a marbled effect.
08 - Bake for 50 to 60 minutes, or until a toothpick inserted in the center exits clean.
09 - Rest in the pan for at least 15 minutes, then transfer to a wire rack to cool completely before slicing.