01 - Preheat oven to 350°F. Grease and flour a 9 x 5 inch loaf pan or line with parchment paper.
02 - Whisk mashed bananas and melted butter in a large bowl. Add sugar, eggs, and vanilla; mix until smooth.
03 - Sift flour, baking soda, and salt in another bowl. Add dry ingredients to wet mixture and mix just until combined—do not overmix.
04 - Mix cinnamon and sugar in a small bowl for the swirl.
05 - Pour half the batter into prepared pan. Sprinkle with half the cinnamon sugar. Add remaining batter, then top with remaining cinnamon sugar. Use knife to gently swirl through batter for marbled effect.
06 - Bake for 50-60 minutes until a toothpick inserted in center comes out clean.
07 - Cool in pan for 10 minutes, then transfer to wire rack to cool completely.