Preparation

1. Preheat the oven to 350°F (175°C). Grease a 9 by 5 inch loaf pan and line it with parchment paper for easy removal.
2. In a medium bowl, mash the bananas until mostly smooth and set aside.
3. In a large bowl, whisk together the melted butter and sugar until combined. Add the beaten egg and vanilla, then whisk again. Stir in the sour cream.
4. Combine the mashed bananas with the wet mixture and stir until blended.
5. In a separate bowl, sift or whisk together the flour, cocoa powder, baking soda, and salt to ensure no lumps.
6. Fold the dry mixture into the wet mixture gently until almost combined. A few streaks of flour are fine; do not overmix.
7. Fold in the chocolate chips, saving a handful to sprinkle on top.
8. Pour the batter into the prepared loaf pan and smooth the top. Sprinkle the reserved chocolate chips over the surface.
9. Bake for 55 to 65 minutes, until the top springs back slightly and a toothpick inserted in the center comes out with a few moist crumbs.
10. Let the loaf cool in the pan on a rack for 10 to 15 minutes. Turn it out and let it cool completely before slicing if possible.