Quark / curd cheese
A fresh, unripened soured-milk cheese, white and creamy to slightly crumbly, mildly tangy.
Quark is in every supermarket in German-speaking countries; in Britain larger supermarkets carry it, and in North America look in Eastern European shops or for farmer's cheese.
Substitute:
Greek yogurt drained overnight in a cloth comes close but is tangier; cottage cheese pushed through a sieve, or ricotta, tastes milder and slightly sweeter.