I am looking for a chocolate filling recipe for a 3 layer fondant covered marble cake. Any recommendations? I don't really like any that I have seen so far.

Chocolate ganache.

1 pound of dark chocolate.  1 cup of heavy whipping cream.

http://confectionarychalet.com/docs/?page_id=319

I second Apti's motion.  i always make extra so I can enjoy the leftovers.  No cake needed . . . Just a spoon :-)!

I love homemade chocolate fudge filling. ..Mmm.  So yummy and a bit more creamy than just ganache. 

http://www.cakecentral.com/forum/t/758827/fudge-filling