Hi there, Eve! This is likely caused by over-baking the brownies a bit, we'd recommend taking them out of the oven 5 minutes sooner next time. The brownies have to be baked far enough that enough moisture is driven off to allow the sugar to recrystallize at the top and help create that shiny crust, but if they're baked too far there's not enough moisture below to keep the crusty bits attached. We hope this can help and happy baking! 

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