Most cookies have top crusts that remain relatively soft and flexible as the cookies set during baking. However, if the top surface dries out before the cookie is finished spreading and rising, it hardens, cracks, and pulls apart, producing an attractive crinkly, cracked exterior. While we found that multiple factors can affect how quickly the top dries out and thus how many cracks are formed (including the temperature of the dough and how rapidly it spreads in the oven), a simple tweak turned out to be key to producing a maximum number of fissures: rolling the balls of dough in granulated sugar before rolling them in powdered sugar.
Coating the cookies with either type of sugar draws out moisture from their surface, promoting cracks by drying out their tops before the interiors set. But granulated sugar does so more efficiently because of its coarse, crystalline structure. As the crystals absorb moisture, some—but not all—dissolve into a syrup. As the cookies continue to bake, the moisture evaporates, and the sugar begins to recrystallize, a process that is accelerated by the undissolved sugar crystals, which act as “seed” crystals. When enough new crystals form, they begin drawing out moisture once again. The upshot: a cookie with a faster-drying surface that is more prone to cracking.
