I've only made ganache a few times but I heat the cream slowly until, actually I don't really know until when I just stop when it looks hot before it boils haha. Then I just tip in all the chocolate that's been chopped up as small as I can bother and stir until it's all melted. I have then used it straight away on a couple of occasions and have also left it overnight to set then warmed it up the day after and used it then. 3 parts chocolate to 1 part cream for white chocolate and 2 parts chocolate to 1 part cream for dark or milk chocolate I think.
Is that any used to you? It was quite a warm day yesterday (well it was here anyway) maybe that's why your buttercream was a bit soft?