Is the following sentence proper use of grammar?
Of what are you talking?
It would be perfect as a haughty, even sneering, question by a Victorian gentleman in response to someone he regards as talking nonsense.
In other words, it is very out-of-date.
What are you talking about?
If you can't fathom what on earth they are going on about, then;
What ARE you talking about? - slight pause after 'what' and really stress the 'are'