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, mix the mashed bananas, melted butter, granulated sugar, brown sugar, eggs, and vanilla until well combined.
04 - Add the dry ingredients to the wet ingredients and mix until just combined. Do not overmix.
05 - In a small bowl, mix the 1/4 cup sugar with 1 tablespoon cinnamon for the swirl.
06 - Pour half the batter into the prepared loaf pan. Sprinkle half of the cinnamon-sugar mixture evenly over the batter.
07 - Add the remaining batter and smooth the top, then sprinkle the remaining cinnamon-sugar over the top. Swirl lightly with a butter knife for a marbled effect if desired.
08 - Bake for 50–60 minutes, or until a toothpick inserted into the center comes out clean.
09 - Allow to cool in the pan for at least 15 minutes, then transfer to a wire rack to cool completely. Slice and serve.