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 until evenly distributed.
03 - Combine the mashed bananas, melted butter, granulated sugar, brown sugar, eggs, and vanilla in a large bowl, mixing until smooth and uniform.
04 - Fold the dry mixture into the wet mixture, stirring until just combined. Avoid overmixing to prevent a dense crumb.
05 - Stir the 1/4 cup granulated sugar and 1 tablespoon cinnamon together in a small bowl until well blended.
06 - Spread half the batter into the prepared pan. Sprinkle half the cinnamon-sugar mixture evenly over the surface, then add the remaining batter and smooth the top. Dust with the remaining cinnamon-sugar and swirl lightly with a butter knife for a marbled effect.
07 - Bake for 50 to 60 minutes, or 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.