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Underbaked cookies are the secret to softness. Using cornstarch in the dough is another secret to softness, as well as the secret to thickness. Using more brown sugar than white sugar results in a moister, softer cookie. Adding an extra egg yolk increases chewiness. Rolling the cookie dough balls to be taller than wider increases thickness. Using melted butter (and slightly more flour) increases chewiness. Chilling the dough results in a thicker cookie.