I can't speak for the fondant, which will rest until assembly on Sunday, but I just made swiss meringue buttercream (and cooked the egg whites to 160 as recommended), to use on my little cake experiment.
Oh my word.
The final assembly is a marble pound with grand marnier flavouring, and I added pure orange extract to the buttercream. It. Is. So. Good.
I'm waiting for Hubby to come home from the lab to test it on the extra cake piece I have hanging around ;)