Preheat your oven to 350°F (175°C). Grease a standard loaf pan with butter or cooking spray.

In a large mixing bowl, mash the overripe bananas until smooth.

Stir in the Greek yogurt or sour cream into the banana mash until fully incorporated.

In a separate bowl, whisk together the all-purpose flour, whole wheat flour, cocoa powder, baking soda, baking powder, and salt.

Gradually incorporate the dry ingredients into the wet banana mixture, folding gently until just combined.

Fold in the chopped chocolate, and optionally, some nuts.

Pour the batter into the prepared loaf pan and smooth the top.

Slice a banana lengthwise and arrange it on top of the batter, then sprinkle with demerara sugar.

Bake for about 60-70 minutes, until a toothpick comes out clean.

Let it cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.