Chocolate ganache is delicious but despite only consisting of two ingredients -- chocolate and cream -- it's difficult to get it right. The trick is balancing the ratio of chocolate and cream to get the consistency you want.
Over at Kitchen Conundrums, Thomas Joseph goes through three different versions of chocolate ganache which are used for various purposes. To turn the ganache into truffles, you would need to use two parts chocolate to one part cream.
A rich chocolate frosting to add onto cakes, you would use an equal ratio of chocolate and cream. For a thinner glaze for things like cupcakes or bunt cakes, you would increase the amount of cream.
You can check out the video where Joseph goes through the science behind the perfect chocolate ganache in more detail.