Hi Tamson… I generally use a white chocolate ganache under my fondant. I am in Australia and use Snowcap, which is now available at specialty decorator stores. I make it using 1 part cream, to 3 parts chocolate. In Australia, it can be found at The Raspberry Butterfly, which has an online store. I LOVE to make a caramel from scratch, which I mix into the white ganache as well, (I love caramel).
I am not the best person to ask about dark choc… I am yet to use some of the better brands to make dark ganache… Julez
Julez, https://www.facebook.com/pages/Allways-Cakes/450634018350115?ref=hl