Preheat the oven to 350℉. Lightly spray a loaf pan with cooking spray and dust with cocoa powder. Set aside.
In a medium bowl, mix together the bananas, sugar, butter, egg, egg yolk, sour cream and vanilla until fully combined.
In a separate bowl, mix together the cocoa powder, flour, baking soda and salt.
Slowly 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, if desired.
Bake in the preheated oven for 60-65 minutes, or until a toothpick inserted into the center comes out clean.