It is funny that it is uncle you once told me it is very much not right to merely say "how to". (To start a sentence with how, how to sing? What to do? Where to go? Why to do?)
It looks like it's no problem to say "I don't know how to do it", "I know how to cook", "I know what/where/why to do/go/etc" as I found these in dictionaries. Though I'm not sure what I should call them. They're not supposed to be called clauses.
And the most interesting part is that which part of speech they (how, the W family) belong to seems like a mess and they all just work in the same way.
They say how works as a conjunction in "I don't know how to...", so could be "where" but you'll see others could not be conjunctions (what could be but not in this case as they say) at all.
Obviously this is not a mistake esl speakers created, I'm sure y'all use that structure no less than us. Well maybe in that case you'd just call it an idiomatic expression not a complete sentence.