Baking Ammonia
Baking ammonia is a leavening agent most often used in Scandinavian baking and is also known as hartshorn powder. The closest substitute for baking ammonia is cream of tartar, which will provide less crispness.
Useful Links:
What is Baking Ammonia? – KitchenSavvy
What is Baker’s Ammonia? | Cook’s Illustrated
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