Don't know what it is but I like the card. The German postcrosser who sent it from Berlin wrote: I hope you like this card. It's kind of different.
Note: My thanks to Henderica, who sent the link in the comment below. I checked it out and sure enough the mystery card seems to fit the profile of chocolate-covered marshmallow treat. I also learned that the first such confection was made in Denmark maybe 200 years ago and that similar treats enjoy near-iconic status in cultures around the world.