Preheat oven to 350°F. Lightly grease two 9x5 inch loaf pans and set aside.
In a large bowl using a hand mixer or in the bowl of a stand mixer with the paddle attached, cream the butter and sugar together until light and fluffy. Add the eggs one at a time, incorporating well after each addition. Stir in the bananas, sour cream, and vanilla until fully incorporated.
In another bowl, whisk together the flour, baking soda, and cocoa then stir into the banana mixture. Mix on low speed until the wet and dry are combined completely. Fold in the chocolate chips.
Pour the batter into prepared pans.
Bake in the preheated oven for 55-65 minutes, or until a cake tester or toothpick inserted into the center of each loaf comes out clean.