You can absolutely do that. Just make sure not to roll your fondant too thin. When I make ganache I use a 1:1 ratio of heavy cream to chocolate (70% cocoa chocolate). This works fine under fondant, although you will also find a 1:2 ratio often quoted (1 part cream to TWO parts chocolate) for if you want your ganache to really be SUPER firm under that fondant.
I would use Toba garrets chocolate frosting recipe. It is so good and easy to cover with fondant.
Yes you certainly can. I myself use the 2/1 ratio for ganache under fondant. 
how do you know how much ganache you will need? I've only ever used it as a filler, not to completely frost, say, a 2 layer 9" cake.
Thanks for the info. i can't wait to try it!
The cake will be taken to the party Sunday evening, but I need to ice and cover the cake Saturday. If I make the ganache Friday night, can I put it on the cake Saturday and it still be oK for Sunday at room temp?
Do I need to tuck the fondant under the bottom of the cake to seal it?
Will the cake be oK outside in warm weather w/ ganache?
Sorry for all the questions. I usually just use white buttercream and do not cover w/ fondant, but the person the cake is for really wanted Chocolate icing, so I thought using ganache and covering with fondant would make it all work.
I have a 10 inch round, two 8 inch rounds, and a 9 inch to do.
slsharratt, I don't know how much you'd need for a 9 inch layer cake. I assume that if you look at the charts for how much icing you'd need, it would be the same amount of ganache.
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