Translation:Do you know which restaurant sells steamed pork dumplings?
Yes, but it usually and traditionally is, the shrimp and other versions are more modern.
i thought you wouldn't need a "吗“ in this sentence because you also have "哪" to indicate it's a question. why does this one have both?
the 吗 goes with 你知道. If there was no 吗 it would just mean "you know which restaurant sells steamed buns"