I think Julia is onto something - my recipe also uses only powdered sugar and no granulated sugar. However, my proportions are very different, with way more butter than sugar, so other factors may account for it as well. I really love my recipe, so am OK with scraping off the occasional flaky tops for the sake of taste, but then again, I'm not a professional baker. I only roll between parchment paper, and find that not rerolling the dough and not letting the dough air dry limits the issues. Let us know if you find a solution by experimenting!