The ultra-tender, extra-crunchy texture of these cookies is unlike anything you can get using baking powder or baking soda. The secret? Baker's ammonia, a.k.a. ammonium carbonate; possibly available at your local pharmacy, and certainly available here at King Arthur Flour. Can these cookies be made without baker's ammonia? Sure; just substitute baking powder. Just be aware their texture won't reflect what we've just described.