What it is
An old-world leavener — once distilled from deer antler ("hartshorn") — still used for extra-crisp thin cookies and crackers.
Why your biology objects
On heating it decomposes completely to CO₂, ammonia and water and leaves no residue, which is why it only works in thin, dry bakes where the ammonia can escape. A pre-industrial leavening that predates baking powder by centuries.