This took place in a AU Season Six, where Spike's had enough of his relationship with Buffy and leaves. He eventually gets the chip out in New York, and comes back to Sunnydale. Buffy has had a baby girl, named Maryan after getting pregnant by the guy Xander tried to set her up with in "Older and Far Away". Spike becomes the baby's nanny, and the unresolved tension between Spike and Buffy ensues.

That's Baby Steps by Kallysten. She turned it into original fiction and, I believe, doesn't want it exchanged? If not, I have a copy.