It was Kurt/Blaine, but at the time I read it I think it was still pre-slash (it was a multi-chapter fic). Kurt and Blaine knew each other as children and are best friends, but somehow lose contact (I don't recall how) and don't meet again until Kurt goes to Dalton to spy.
I remember there was a scene based on the scene where Blaine, Wes, David and Kurt have coffee, and Blaine knows at this point that he used to know Kurt. At the time I read it, it was not complete.
I know there's a few fics based on this premise, but I'd really like to find this one again. Can anyone help?
(Mods, I've tried tagging but it won't add them for some reason, I don't know why.)