Yes, it's the ingredient that stumps you. Hartshorn is also known as baker's ammonia and ammonium carbonate. It is an old time leavening that literally used to be made from ground deer horn. Hart is the German work for deer and that is how it is so named. It is not commonly found in everyday groceries. Years ago I would purchase hartshorn (and also anise oil) at a pharmacy; that was back when pharmacies did compounding and mixing right in their shops. Not something easily found these days. Thus, many people started substituting baking powder in their Springerle. I still really like hartshorn as it produces a fluffier, lighter and softer texture in Springerle. Some Springerle lovers grew up with the baking powder version and actually prefer the harder cookie that is the result....and that's just fine.
But if you decide to try the hartshorn, here are some things you should know. Firstly, it stinks!! Yes when you open the jar it smells strongly of ammonia, which makes sense since it's ammonium carbonate. Secondly, it will readily evaporate if you do not keep it tightly sealed (also why it's aroma is so strong). Also, you should NOT eat the raw dough as it will give you a frightful and painful case of flatulence. This is probably another reason that it is not as commonly used anymore. After all, it is difficult to keep the cookie monsters at bay. The ammonia dissipates when you bake the cookies. You can substitute hartshorn 1 for 1 for baking powder in other cookie and cracker recipes. Don't use it in breads, cakes, or rolls, only in baked goods that are thinner and fully baked, so that you know that the ammonia is completely gone. You might want to try it in a sugar cookie recipe, just for fun, to see the difference in texture.