Preheat the oven to 350 degrees. Spray a 8-inch loaf pan with cooking oil spray and set aside while you prepare the batter.
In a large bowl, stir together the butter and sugar until well-combined. Add the egg and stir well to combine. Stir in the mashed bananas.
In a medium bowl, sift together the flour, baking soda and salt. Add the sifted dry ingredients to the wet ingredients and stir until fully incorporated -- but be careful not to over mix. Once it's all moistened, stop.
Pour half of the batter into the prepared loaf pan. Top with ½ of the cinnamon sugar. Add the remaining half of the batter and cinnamon sugar to the pan. Use a knife to cut a grid pattern through the batter (this will create the swirl of cinnamon through the loaf).
Bake for 50-60 minutes, or until a cake tester (or knife) inserted in the center of the loaf comes out cleanly. Let cool before slicing.
Recipe by Sarah's Cucina Bella at http://sarahscucinabella.com/2015/06/11/cinnamon-swirl-banana-bread/