PuffCake, I made a chocolate cake last week that I frosted with regular buttercream, then filled with Nutella mixed with a little of the buttercream. It was awesome -- people raved about it!!! I haven't tried it with chocolate frosting, but you could definitely taste the hazelnut in mine.
Also, can I just say that I'm always a day late and a dollar short?!! I can't believe that someone beat me onto YouTube with the Nutella song.