I have seen these beautiful panache covered cakes and would love to make one but I have a question. I thought that ganache hardens. Does this make the cake have a hard shell? 

Depends on your ganache recipe.

The ganache I use under fondant  is a ratio of 2 parts chocolate to 1 part cream, so it sets up firm. It can be sliced. It is not a hard shell which shatters, like if you used pure chocolate.

If I'm making a poured ganache, I use closer to a 1:1 ratio and the ganache is softer again. But that's not to put under fondant - it would be used to finish a dessert cake.

Sorry I can't help with a ganache recipe or method. My wedding cake was a mocha cake with chocolate filling & chocolate icing with ganache poured on top. It was absolutely delicious & soft, we even ate it the next morning for breakfast!