A compound also known as hartshorn powder that was once used as a leavening agent. It's most often used in Scandinavian baking and is available at pharmacies and through mail order. Cream of tartar is an acceptable substitute, although cookies made with it are less crisp than those made with baking ammonia. If you use baking ammonia for baking, use caution when opening the oven door because irritating ammonia-like fumes may be produced.