Preheat your oven to 350°F (175°C) to ensure even baking.
Grease your loaf pan with vegetable oil or line it with parchment paper.
In a mixing bowl, whisk together the all-purpose flour, baking soda, and kosher salt until well combined. Set aside.
In another bowl, combine the mashed bananas, vegetable oil, granulated sugar, and egg. Mix until smooth, then fold in the sour cream.
Gradually add the dry ingredients to the wet mixture, stirring gently until just combined.
Fold in the dark chocolate chunks and toasted walnuts until evenly distributed.
Pour the batter into the prepared loaf pan and smooth the top. Bake for 55-65 minutes or until a toothpick comes out clean.
Remove from oven and let cool in the pan for 10 minutes before transferring to a wire rack to cool completely.
