First of all, I've been fiddling with my personal archive so that's why I've been coming across these today. It's not a day long search for this particular request.
Second, THIS is the story I've been trying to think of since I saw this request. I am so glad I ran across it because I think it meets your criteria perfectly. Plus, I always liked it myself.
The More Things Change by flibblehttp://spikeluver.com/SpuffyRealm/viewstory.php?sid=17715&warning=NC-17
Summary: Buffy and William are best friends about to embark on their last year of high school. Seen all their lives as nerds Buffy decides they should both use the summer to transform themselves into 'date worthy material'. How will this joint transformation go? And how will they react to each other when they suddenly become all hot and sexy?