Preheat oven to 350 degrees F. Grease and flour an 8½x4½-inch loaf pan.
In a small bowl, toss the chocolate chunks and walnuts with 2 tablespoons of the all-purpose flour.
In a medium bowl, sift together the remaining flour with the baking soda, then stir in the salt.
In a large bowl, whisk together the mashed banana, oil, sugar, egg and sour cream until completely smooth. Add the dry ingredients and stir until just combined. Fold in the chocolate chunks and walnuts until evenly distributed.
Transfer the batter to the prepared pan and bake until dark brown and a toothpick inserted in the center comes out with a few moist crumbs attached, 60 to 75 minutes. Place the pan on a wire rack and cool for 15 minutes, then turn the bread out of the pan and allow to cool to warm room temperature before serving. The bread can be stored, wrapped tightly in plastic wrap at room temperature, for up to 5 days. This bread can also be frozen (wrapped in plastic wrap and placed in a freezer ziploc bag) for up to 2 months.