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 a separate bowl. Add dry ingredients to wet mixture and mix just until combined.
04 - Mix cinnamon and sugar together in a small bowl.
05 - Spread half the batter in the pan. Sprinkle with half the cinnamon sugar. Add remaining batter, then top with remaining cinnamon sugar. Use a knife to swirl gently for a marbled effect.
06 - Bake for 50-60 minutes until a toothpick inserted in the center comes out clean.
07 - Let bread cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.