Is it possible to walk across the border from Hong Kong into China and come back?

I'm going to Hong Kong for work. On the last day, I have the whole day free before flying out of HK late at night. All I want to do is to pass through the Chinese customs, get a stamp in my passport, then immediatly walk back into Hong Kong. What's the fastest way to do this?

I do have a valid entry visa for China in my US passport.

Thanks for your help!