This delicious soft icing is a little tricky to make, so follow the instructions exactly. Quick and accurate decisions are necessary in judging when the icing is ready and then it must be applied to the cake immediately. If the icing is not cooked enough, it will still taste good, but will not dry out properly on the outside. If cooked too much, it will be difficult to spread over the cake. Any leftover walnuts will keep best in the freezer. Like many cakes with nuts in them, this cake keeps very well if properly stored for a couple of weeks.