Pfeffernüsse cookies, the frosting
Confectioners sugar, egg white and lemon juice. The judges are out on whether the eggwhite should be beaten stiff or only mixed roughly with a fork. Your picture would point to liquid egg white, but then Bahlsen is factory stuff, so all bets are off.
Personal opinion: this may look good, but I prefer a simple confectioners sugar and lemon juice without the egg white for taste and mouth feel. Apply when the Pfeffernüsse are still warm.
I have always seen them covered in powdered sugar, but here is a blog where powdered sugar was mixed with a bit of lemon juice to form the glaze.
Another site which offers a recipe for sugar glaze: