**Yes—you can use quark, but expect a noticeable texture change, more than a drastic flavor change.** Quark is used in baked cheesecakes, though this particular recipe was developed for cream cheese. [Nigella’s cheesecake guidance](https://www.nigella.com/ask/cheese-for-cheesecakes)

One important detail: **“40%” on quark usually means fat in the dry matter**, rather than 40% of the whole product. For example, MILRAM’s 40% quark contains about 10 g fat per 100 g, while Philadelphia block cream cheese contains about 36 g per 100 g. Check your tub’s nutrition panel. [MILRAM](https://www.milram.de/produkte/quark-sahne-schmand-co/sahnequark-40-prozent-fett/), [Philadelphia](https://www.kraftheinz.com/philadelphia/products/00021000612239-original-cream-cheese)

Given that difference, I’d expect:

- **Flavor:** a little tangier and less buttery. The sugar, vanilla, cream, and browned surface should still give you a familiar cheesecake flavor.
- **Texture:** less silky and rich, with more of a curd-cheesecake texture. Depending on how wet your quark is, it could release some whey; baking it too long could make it grainy or dry.

If you want to try it with what you have, I’d use **2 lb / approximately 900 g of thick, plain quark**. If it has visible whey or is loose, drain it in a cloth-lined sieve in the fridge first, then weigh it. Keep the other ingredients as written for a first attempt—including the heavy cream—and bake until the top is deeply browned but the center still jiggles. Let it cool completely before judging the set.

That’s a reasonable adaptation, **not a tested equivalent**. It can make a tasty cheesecake, but if the creamy Basque texture is what you especially want, getting cream cheese is the more reliable choice.