This an AU human fic but it's a fave of mine. It's by a great author
If you enjoy younger Buffy and older Spike you should enjoy it.
It's not that long. Just 5 chapters.
Plus there is plenty of smut.

Little Miss Not So Innocent by OKDeanna
http://www.nocturnal-light.net/fanfiction/story/540/Little_Miss_Not_So_InnocentSummary: All human AU. Forced by his best friend to take his kid sister to her senior prom, Spike Pratt decides to make the most of it and show her a good time. But when the good girl he picked up as his date somehow becomes the bad girl of his dreams at the dance, he finds himself torn between doing the right thing and doing the wrong thing at exactly the right time. Complete