01 - Preheat oven to 350°F. Grease and flour a 9 x 5 inch loaf pan or line it with parchment paper.
02 - In a large bowl, whisk together mashed bananas and melted butter. Add sugar, eggs, and vanilla; mix until smooth.
03 - In another bowl, sift together flour, baking soda, and salt. Add dry ingredients to wet ingredients and mix just until combined—do not overmix.
04 - In a small bowl, mix together the cinnamon and sugar for the swirl.
05 - Pour half of the banana bread batter into the prepared loaf pan. Sprinkle half of the cinnamon sugar mixture evenly over the batter. Top with remaining batter, then sprinkle the rest of the cinnamon sugar mixture over the top.
06 - Use a knife to gently swirl the cinnamon sugar layer through the batter, creating a marbled effect.
07 - Bake for 50–60 minutes, or until a toothpick inserted in the center comes out clean.
08 - Allow bread to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.