As mccfluff says,mix cream into it,making a ganache.It will at first look like something has gone wrong,but keep mixing.Single or double cream works just as good as each other,a small knob of butter will a a little sheen to the finished topping.You can make it fairly soft,as far as I know there are no hard and fast rules as to how much cream you can add,but it also is best to mix in a bowl,then let it set in the fridge for half an hour before spreading.
Enjoy a yummy cake! Aya