I think I'm mixing up several fics but I'm looking for one where in an alternate universe Spike jumps through a portal to get Buffy back from what is the final battle in AtS and grabs the wrong one. Everyone follows them back or maybe Grown up Buffy helps rescue baby Buffy.

I'm thinking it is the evil Willow one, but I can't remember. ...

This made me think of Ripping Down the Walls by Peta but it is so long since I read this fic it may not be the right one.

Yay!!! That's the one. You're a gem!