"What are you doing?"
You can, but it means something completely different – „jaki” really means something a lot more like „what kind of” than „which” or „what”, so „jaki robisz” is a question about what kind of [something] you are making… For example:
- –Robię tort.
- –Fajnie! Jaki robisz?
–I'm making a cake.
- –Cool! What kind of cake?
- –Chocolate one.
I once wrote a comment about it:
"Który is asking 'which one of the finite set of choices is the one' while Jaki is more like "what kind of":
"Którego psa brakuje?" – Is asking which dog is missing; most logical context would be, I don't know, like aftermath of a flood in the kennel and the expected answer would be "Azora"(a dog name).
"Jakiego psa brakuje?" – Is asking what kind of dog is missing; most logical context would be if you are preparing for a dog show and you have a list of dog breeds you want on that show, so you ask which breed is still missing; expected answer would be something like "Sznaucera"(A dog breed).
Obviously, there is certain overlap between the two, but generally they ask about somehow different things."
And now that you made me think about it some more, I think "jaki" simply asks about adjective – even when it seems to ask about noun, it really wants adjective + noun answer(but since we like to drop the obvious from the sentence, frequently with such a question you will get either the noun or the adjective as answer, not both).
BTW, since I realised now I haven't made it obvious in the previous comment – the "jaki" in the previous example is in accusative, so for feminine nouns it would be „jaką robisz” and for neuter „jakie robisz”.