I have done them a few times attaching onto a fondant covered cake, but like you said, I have seen it directly to the buttercream. I think you shoud do whichever you are more comfortable with. I personally don't work with buttercream all that often, mostly ganache and fondant. That is my comfort zone, so that is what I do. Do what you feel comfortabe with, since it seems both methods work.
6/25/12 at 12:24pm