We did this exactly as the recipe describes and we also got what looks like grey asphalt. I might try it again with maybe 1/4 as many crushed cookies and stir them in once the chocolate is melted and spread in the pan, perhaps that would look remotely like the nice white “bark” in the photo.
Healthy chocolate brownies
Heat the oven to 190C/170C fan/gas 5 and line a shallow 23cm square baking tin with baking parchment. Tip the beans, cocoa, coffee granules, xylitol, vanilla, eggs and yogurt into a large bowl. Blitz using a hand blender until smooth. Or, do this in a food processor. Add the baking powder, then blitz again.