Preheat the oven to 350℉. Lightly spray a loaf pan with cooking spray and dust with cocoa powder. Set aside.
In a large bowl, combine the banana, sugar, butter, egg, egg yolk, sour cream and vanilla until smooth.
In a separate bowl, mix together the cocoa powder, flour, baking soda and salt.
Mix the dry ingredients into the wet ingredients until just combined. Fold in the chocolate chips.
Pour the mixture into the prepared loaf pan. Top with additional chocolate chips and bake in the preheated oven for 60-65 minutes, or until a toothpick inserted into the center comes out clean.