"Do you know which restaurant sells steamed pork dumplings?"
Do we really need the 'ma' at the end if there is already a question word ( na3 ) in the middle?
We do for yes or no questions. In this case, we're not asking "which", we're asking "do you know?", which then requires a 吗 at the end.
Is this really a yes or no question though? Grammatically yes, but in practice this question in English is asking for directions to a store that sells dumplings like 99% of the time. The question is, is the Chinese sentence wrong grammatically without the 吗 or does it simply have a different meaning?
In the English translation the question isn't which shops sells the dumplings, but whether or not you know which shop does, so in that case there needs to be a 吗 at the end. But this is just a technicality and in real life you will get to the dumpling shop whether you ask "Which shop sells dumplings? 哪家饭馆卖小龙包？" or "Do you know which shop sells dumplings? 你知道哪家饭馆卖小龙包吗？"
In real life if somebody asks you this in English and you answer "yes" or "no", the very least that will happen is that you'll be considered a smartarse.
It's actually a strange form of politeness in English that you ask certain questions in a roundabout way rather than directly. This is more pronounced in British English than American English but exists in both.
It's the same way we say "Could you pass me the salt?" instead of "Pass me the salt!".
Steamed pork dumplings is ambiguous here, in English this can equally be applied to xiao mi, jiao xi and other steamed pork dim sum. In the UK at least we use romanised versions of the Cantonese names.
In Australia we used romanized versions of the Cantonese names for Cantonese foods that we got accustomed to over the decades, but for non-Cantonese foods or foods that have been becoming popular here since China has been getting rich, we'll use the romanized Mandarin words sometimes.
And then there's "cha siu bao" which is spelled various ways and contains barbecued pork but in a bun shich is steamed (-: