Preheat oven to 350F. Grease and flour a standard loaf pan. In a small bowl, toss chocolate chunks and walnuts with 2 Tbsp flour. In a medium bowl, sift together remaining flour with baking soda, then stir in salt.

In a large bowl, whisk together banana, oil, sugar, egg, and sour cream until completely smooth. Add dry ingredients and stir until just combined. Fold in chocolate chunks and walnuts until evenly distributed.

Transfer batter to prepared pan and bake until dark brown, 60-75 minutes. Place pan on a wire rack and cool 15 minutes, then turn bread out of pan to cool.

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 bag) for up to 2 months.