I always thought Amerikaner were a German thing and wondered where they got their name from, but it turns out they are a New York cookie (via smitten kitchen).
(Image by Ben Orwoll, public domain)
Amerikaner certainly used to be made with a form of ammonium carbonate called Hirschhornsalz (Salt of Hartshorn/baker’s ammonia) in German. This is widely sold in Germany, especially at this time of year. I saw it being used by London Eats, who posts Christmas cookies from abroad at this time of year: Fedtebrød.
If you don’t want to make do with baking powder or bicarbonate of soda, the German Deli sells Hirschhornsalz, and also potash and Lebkuchen spice.
As an American who never saw an Amerikaner until he came to Germany, I would say they are a German thing. I once heard that they got their name from the fact that, like the soldiers who occupied Germany after World War II, they were partly white and partly black. But that may be just a story.
I thought that was how they got their name too. But I am not sure. Or perhaps like Feirefiz: https://en.wikipedia.org/wiki/Feirefiz
Perhaps it was because I grew up in the British zone in post-war Germany that as a young child I never knew ein Amerikaner could also be human and not edible. Then one day a man walked along the street in shorts and created quite a stir. Then a woman found an explanation. “Das ist bestimmt ein Amerikaner!” I was utterly amazed.