A hot charlie is anything you want it to be. Because of the way it sounds, most people (especially girlfriends) associate it with something sexual (i.e. Dirty Sanchez). Simply ask for a hot charlie, and the other person must determine what his/her definition of it actually is.
Me: Hey, I'm going to ask my girlfriend if she wants to give me a hot charlie tomorrow night after the movies.
Friend: What the crap is a hot charlie?
Me: Oh, I think you know.
Friend: Does it involve an indian burn on the thigh?