Well, before I say anything I'll preface my response with the same thing that I preface my responses with EVERY TIME I see a relationship advice thread. That being that none of us responding, myself included, have any idea what the McFuck what we're talking about, so you should take any advice we offer with a massive heaping spoonful of salt. This is because we don't actually know the people involved. Any "advice" (and I use that term loosely) that we can offer is just general advice. But it's not good advice. Relationship counselors generally spend their first meetings with any couple, not offering advice, but just getting to know the couple. And this is why we can't offer good advice, because we don't know you, this girl you're best friends with, or this girl you're best friends with's grandmother's dog. We know nobody. All we know are a couple lines of text briefly outlining the situation. The people who are best suited to give you actual good advice on this situation, are already involved in it.
That said, we can offer general advice, and what I would say is that any girl who knows that you're into her, has made it clear that she's not into you, and is doing stuff like this with you anyways... not only is she not a good friend, but she's not going to be a good lover either. There are a couple reasons why someone in her position might do something like that, just to name a few, it could be that she herself is a confused emotional mess of a person (in which case you might actually have a shot, though even if you do, you're buying a ticket for one hell of a bumpy ride), it might be that she's kind of a ***** and she's messing with your head for funzies, or it might be that she's feeling sorry for you and is too naive to realize that kissing her won't alleviate your desires, only make them worse. So, in one situation you're dealing with someone who's going to be an emotional tornado for the entirety of the time you know her, in another you're dealing with someone who's kind of a *****, and at best you're dealing with someone who cares about you as a friend and is to naive to realize that she's only making things worse. In the first situation, you'd be better off just staying friends to start with, and if there's any chance for something more she will probably present it to you. In the second, well, that's not too hard to figure out, she's just playing with you and you're gonna get burned. In the third if you approach her all you're going to do is dammage further what already sounds like a confused relationship. My advice, let it go. If something's gonna happen, it'll happen on it's own, and if it's not then you'll only make things worse for yourself.