Ganache is the only thing I use under fondant you can make an awesome white chocolate ganache. I personally find buttercream too unstable and only use it if i absolutely must. Ratio is 3:1 or 4:1 depending on humidity. Being chocolate:cream ration. I normally make the ganache 24 hours before I need to use it. It will stabilise and become thick and spreadable. Then after covering the cake allow to sit overnight to form a crust. Then cover with fondant. This technique is used by nearly every cake decorator I know of.