The cake is a very rich, dense chocolate cake. What filling would be good that would enhance the flavour but not be too sickening? I thought either a vanilla buttercream or cookies and cream.

Filling suggestions for chocolate cake:

http://forum.cakecentral.com/cake-decorating-ftopicp-1826261-.html

HTH

In my opinion, you can't beat raspberry filling when it paired with chocolate. I also love mocha buttercream. Caramel is another very popular flavor with chocolate. Yum!!!

As much as I like raspberry filling, it's a distant second to lemon curd filling. It's absolutely fantastic with any type of chocolate cake.

raspberry is my favorite

I have to go with rasberry as well. There is just something so wonderful about choc and ras. I would suggest the "Rasberry mousse filling 2 in the recipe section. If it is a multi layer cake you could make just the middle layer the rasp. the other layers choc. On the rasp layer I'd sprinkle lighly with a little Chambord and maybe put a layer of fresh ras on top of the mousse.
Have fun

I like orange mousse or hazelnut buttercream.

peanut butter filling