Hi Laura, Baking ammonium, also called ammonium bicarbonate, is a leavener used in baked goods before baking powder and baking soda became popular. It is not at all like household ammonia. You will still sometimes see European cookie recipes use baking ammonium; it makes cookies and gingerbread very light and crisp. You may be able to find baking ammonia in drugstores; it comes in lumps, which you need to grind into a fine powder to use in a recipe. If you can’t find baking ammonia, you can substitute an equal amount of baking powder, but the final outcome may not be as light or crisp as desired.