Marbled Banana Bread

Name: Marbled Banana Bread
Prep time: 20 min
Cook time: 70 min
Total time: 90 min
Yield: 1 ({9 x 5-inch|23 x 12.5-cm}) loaf
Ingredients: 1¾ cups all purpose flour, spooned into measuring cup and leveled-off; ¾ teaspoon baking soda; ¾ teaspoon salt; 1 cup sugar; 1 stick (½ cup) unsalted butter, softened; 2 large eggs; 1 cup mashed very ripe bananas, from 2-3 bananas; ½ cup sour cream; 1 teaspoon vanilla extract; 3 ounces semisweet chocolate, chopped (a heaping ½ cup); 2 teaspoons natural unsweetened cocoa powder, such as Hershey's