Preheat oven to 350F. Grease a 9x5 inch loaf pan with butter or nonstick spray, then line with parchment paper leaving a 2-inch overhang on the long sides for easy removal.

In a large bowl, mash the bananas thoroughly until mostly smooth with small lumps remaining. Whisk in the melted butter, eggs, vanilla, and sour cream until well combined.

In a separate bowl, whisk together the flour, cocoa powder, both sugars, baking soda, and salt until no cocoa lumps remain. Sift through with your fingers if needed.

Pour the dry ingredients into the wet. Using a rubber spatula, fold together until just combined with a few streaks of flour remaining. Add 3/4 cup chocolate chips and fold in gently. Do not overmix - the batter should be thick and slightly lumpy.

Transfer batter to the prepared pan and smooth the top. Sprinkle remaining 1/4 cup chocolate chips evenly over the surface. Bake 55-65 minutes until a toothpick inserted in the center comes out with moist crumbs but no wet batter. The top will have a slight crack and feel set when gently pressed.

Cool in the pan for 15 minutes, then use the parchment sling to lift out onto a wire rack. Cool completely, at least 1 hour, before slicing. The texture sets as it cools.