Preheat the oven to 350°F. Grease a 9 x 5-inch loaf pan generously with butter or nonstick cooking spray.
In a medium bowl, whisk together the flour, baking soda and salt. Set aside.
In the bowl of an electric mixer, beat the butter and sugar together on medium speed until light and fluffy, about 2 minutes. Add the eggs one at a time, mixing well after each addition. Reduce the speed to low, add the flour mixture, and mix just until combined. Add the mashed bananas, sour cream, and vanilla extract, then mix briefly until everything is incorporated. Avoid overmixing to keep the batter light and tender.
Place the chocolate in a microwave-safe bowl and heat on high in 20-second intervals, stirring after each one, until about 80% of the chocolate is melted. Stir until completely smooth, allowing the residual heat to melt any remaining pieces. Add the cocoa powder and mix until fully dissolved. Then, stir in 1 cup of the banana bread batter until well combined and smooth.
Spoon the plain and chocolate batters alternately into the prepared loaf pan, creating a marbled effect. Use a knife to gently swirl the two batters together—about 4 to 5 passes up and back is perfect. Avoid overmixing, as it will blur the colors. Bake for about 1 hour and 10 minutes, or until a cake tester or toothpick inserted in the center comes out clean. Let the loaf cool in the pan on a wire rack for about 10 minutes, then carefully remove it from the pan and allow it to cool completely on the rack.
