In the fic, called Forgive Me, Chris had a daughter with Darren before glee started, and at first when he finds out that his character, Kurt, is getting a boyfriend all he knows about the actor is that his name is Darren. With the help of Dianna(I believe) he pieces together that there aren't very many actors/singers named Darren in that age range, and he eventually figures out that it is, indeed, Darren Criss.
I do think that it was a multi-chap, but at the time I had only read the first chapter.
If anyone can help find this fic or the author, thank you so much in advance!!!
(I hope I got all of the tags right.)