No, no ganache! I've been reprimanded before 
... no, seriously, I posted a whole thread about this about a week ago. I had some malfunction on one of my cakes, and ended up with melting fondant pooling on the front of my cake board. Several very smart people answered, including Sharon Zambito (Sugarshack), and warned me to absolutely not pour my ganache over buttercream, saying that ganache would be the crumbcoat itself (and you can keep on coating until you're satisfied). Good luck!
-Emi.
I ALWAYS cover my cakes in a thin coat of american buttercream first. Vanilla or chocolate, whichever you prefer. I then refridgerate the cake for at least 30-40 minutes to let it get cold. Pour the ganache over the cold cake. I NEVER had a problem with the buttercream melting under the ganache.
You do realize ganache is it's own frosting. You have to let is set up to get to frosting consistency, but it IS a frosting. YOu crumcoat with it & go back & frost with it lke any other butter crea, Why on earth would anyone put butter cream underneath first? 
Shes talking about a loose ganash glaze, the runny kind rather than a thick frosting consistency. it gives a really smooth polished finish to the cake when set, but you do need a good smooth layer of thick, preferably crusting ganash/buttercream underneath that, otherwise it looks messy.
Its down to prefrence, I have done ganash over ganash this way, and ganash over buttercream this way. The only tip with the buttercream is to let is set and refridgerate for a long time, and make sure your ganash isnt hot when you pour it.
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