Preheat your oven to 350°F (175°C) to ensure even baking.
Grease a 9x5 inch loaf pan with cooking spray or line it with parchment paper.
In a medium bowl, whisk together the all-purpose flour, baking soda, and kosher salt. Set aside.
In a large mixing bowl, combine the mashed bananas, vegetable oil, granulated sugar, and the egg. Whisk until smooth.
Fold in the sour cream into the banana mixture until fully incorporated.
Gradually add the dry ingredients to the wet mixture, stirring gently until just combined.
Gently fold in the dark chocolate chunks and toasted walnuts.
Pour the batter into your prepared loaf pan and smooth the top. Bake for 50-60 minutes.
Once baked, cool in the pan for 10 minutes before transferring to a wire rack to cool completely.