Heat the oven to 350°F (175°C). Grease a 9 x 5-inch loaf pan and line it with parchment paper, leaving an overhang on the long sides so the baked loaf can be lifted out.

Whisk the flour, cocoa powder, baking soda, and salt together in a medium bowl until evenly combined.

In a large bowl, whisk the melted, slightly cooled butter and brown sugar. Add the eggs and vanilla, then whisk until the mixture is smooth and well combined.

Stir in the mashed bananas and sour cream or plain Greek yogurt until the wet mixture looks evenly blended.

Mix in the melted chocolate, distributing it throughout the banana mixture.

Add the dry ingredients and fold just until no dry streaks remain. Avoid excessive mixing, since it can make the loaf tough or rubbery.

Fold in about 3/4 cup of the chocolate chips. Transfer the thick batter to the prepared pan and scatter the remaining chips, plus the optional additional chips, over the top.

Bake for 55–70 minutes, beginning to check at 55 minutes. The loaf is ready when a toothpick inserted in the center shows moist crumbs and perhaps melted chocolate, but no wet batter; check more than one spot because chocolate chips can make a toothpick appear wet.

Cool the loaf in the pan for 15–20 minutes. Use the parchment overhang to transfer it to a rack, then cool completely for at least 1 hour before slicing.