Instead of baking powder, many Scandinavian cooks use hartshorn, a.k.a. "hornsalt" or "baker's ammonia" or "ammonium carbonate" as a raising agent for delicate baked goods like cookies and crackers. It's a mystery to me as to how they ever discovered it, for this fine white powder was originally produced from deer antlers, hence the name ("Hey Siggi, give me that antler! I bet if I grind it up and add it to my dough, it'll make my cookies rise better!").
No matter how it was discovered, though, hartshorn salt does in fact create a superior texture in cookies like Serinakaker (Norwegian Butter Cookies), without imparting the sour aftertaste that sometimes happens with baking powder or soda. Because it works by releasing ammonia gas activated by the heat of baking, there is a smell of ammonia while one's cookies are in the oven, but this dissipates quickly and doesn't affect the flavor (so don't be tempted to sample your cookie dough. Hartshorn salt also shouldn't be used in heavy baked goods like cakes, which are too dense for the ammonia to disappear completely).
Discover hartshorn for yourself: it's fairly easy to find in ethnic food markets, is occasionally available as "ammonium carbonate" in drug stores (but should be ground to a powder if used in this form), or can be ordered online from suppliers such as Ingebretsen's.
Hornsalt image & copy;©2010 Kari Diehl, licensed to About.com.