It depends on the meaning you are trying to convey. I can see contexts where one variant (As smart as...) is correct, and another context where the other (However smart ...) is.
If the person you are are addressing really is smart (in their view and, more importantly, yours) but has done or said something stupid, it would make more sense to say 'As smart as you may be, you can at times be a real idiot.' The 'may' is this context actually means 'are'. [Of course, politeness would prevent you from ever calling someone an idiot ... at least, to their face!]
If you don't know whether they really are smart, then it would be more appropriate to say 'However smart you may be, I am smarter. [Not that a polite person would ever be so boastful.]