Takes place post-Ats, willow is running a school for slayers and spike is one of the teachers and he's unoficially adopted one of them. Buffy comes to the school to teach and she and Spike interact, I think Spike has amnesia or something because he doesn't really remember her.

I remember that it's a sequel to a shorter fic but I can't recall the author or title.

It's by Miss Murchison. I can't remember the title right now, but if you look her up on LJ you'll probably find it.

ETA: Found it. The first fic is called Sweet Lethe, and the sequel is Forget and Smile.