Every chocolate cake and buttercream I try to make from scratch is always way to light brown and the taste is never a yummy chocolate taste
For buttercream I use Hersey's Special Dark Cocoa and it makes a very good dark chocolate buttercream.
Try using dark chocolate and you can always add brown food coloring if you want it darker.
I just made a 3D bear the other day, and I used the Hershey's dark chocolate cocoa and it came out really dark! You can see the photo in my album. But if you needed it to be darker you could always add a gel paste brown food coloring to your frosting. =)
What recipe are you using? The recipe you use will make a huge difference in the final product. For a very dark chocolate cake you could try the following recipe:
I've had great sucess with my all crisco buttercream and the Special Dark Cocoa.