Scandinavian cookies often call for hjortetaksalt (Danish), which is translated as hartshorn salt, or just hartshorn which is Baker's Ammonia (Ammonium Carbonate) -. The only place I've found it in the U.S. is from King Arthur Flour on-line.
Icelandic Air Cookies: "The point of these cookies is exactly as their name suggests – they should be light, light, light. They are a doddle to make – icing sugar, cocoa powder and egg. But the magic is the raising agent – ammonium carbonate – which means they puff up spectacularly. As you can see – a six-fold increase in volume!"
Hartshorn salt (ammonium carbonate), also known simply as hartshorn, and baker's ammonia, was used as a leavening agent, in the baking of cookies and other edible treats in the 1800's & still used today .A half-teaspoon of hartshorn can substitute for 1 tsp. of baking powder. It is used in old German and Scandinavian recipes. it helps molded cookies such as Springerle to retain their intricate designs during baking and keeps cookies from hardening for long time. See more on web. by melody