Preheat oven to 350 degrees. Coat an 8½ by 4½ inch loaf pan with cooking spray.
Lightly spoon flour into measuring cups and level with a knife. Combine the flour, baking soda, and salt, stirring with a whisk; set aside.
Place sugar and butter in a large bowl; beat with a mixer a medium speed until well blended. Add banana, eggs, sour cream, and vanilla extract; beat until blended. Add the flour mixture and stir with a spoon, just until moist.
Place chocolate chips in a microwave safe bowl and microwave on high for 1 minute or until nearly melted. Stir until smooth. Cool slightly. Add 1 cup batter to the chocolate, stirring until mixed.
Spoon the chocolate batter alternately with the plain batter into the prepared loaf pan. Swirl the batters together using a knife. Bake at 350 degrees for 1 hour and 15 minutes, or until a wooden pick inserted in the center comes out clean. Cool for 10 minutes in pan, then remove loaf and cool completely on a wire rack.
Notes
Watch the loaf carefully during the last 15 minutes of baking. To prevent the top of the loaf from becoming too dark, you may cover the loaf with foil for the last 15 minutes of baking time.
Recipe by Laura's Cozy Kitchen at https://www.laurascozykitchen.com/marbled-chocolate-banana-bread/