As is so often the case with cookies, the secret is in the sugar. Crackly tops give cookies visual texture and a homey look. Some cooks swear by the cream of tartar in their snickerdoodles, while others insist that shortening, rather than butter, is the key to crackling. In fact, this charming coat of cracks and crevices stems from the multiple chemical reactions that comprise the baking process.
A whole lot of things happen at once when you put a pan of cookies in the oven. Fat melts, sugar melts, air bubbles from leaven expand, flour dries and eggs coagulate, in rapid sequence. Oven heat triggers a race among ingredients that lets cookies spread, puff and dry into final crunchy-on-the-outside, soft-on-the-inside form. When dough is fairly dry and high in fat, as in shortbread, cookies will be consistently dry throughout, with smooth outer surfaces. When cookie dough is moist, outer surfaces may be lumpy or smooth, depending on a number of factors.
To make a crackly cookie, start with a moist dough that contains enough flour to be rolled into balls. Mixed and baked quickly, with a higher fat content than breads, cookie dough stays "short," never permitting flour to form long elastic gluten fibers. This means cookies remain crumbly; the cracks and fissures along the edges of fork-flattened peanut-butter cookies show how short dough can separate in pieces.
Crackling will occur when the outside of a cookie has solidified while the moist inside is still baking. Expanding air bubbles force cracks to let dough continue expanding, in response to the heat. Amplify this reaction by chilling dough before baking. A period from one hour to overnight will chill dough enough that centers lag substantially behind outer surfaces in baking, swelling to force cracks.
Highlight natural cracking tendencies by rolling cookies with granulated or powdered sugar before baking. A sugar coating dries the surface of the raw dough ball, especially if you use powdered confectioner's sugar, which contains cornstarch. Roll light-colored snickerdoodle dough in cinnamon-sugar or colored granulated sugar to emphasize pale cracks. Make dark crackles stand out in chocolate or ginger cookies with a powdered sugar coating.