Hi, here is a recipe copied from a no longer working link a few years ago..apologies if it is a repost.
Grandma pearls flaky chocolate icing
Instructions:
This is what the old folks refer to as a “boiled icing.” This means they make it in a pot on top of the oven and it is brought to a boil until it thickens. It is very much like making fudge, but we use a timer instead of a thermometer to trust it gets to the right temperature. Have you ever had your granny make one of those cakes that when she cuts into it, the icing cracks and breaks off in huge fudgy chunks? This is that icing. This icing hardens very quickly, so it’s easiest to use on a sheet cake, where you simply pour it over the cake while it’s still in the pan and allow it to cool. If you’d like the peanut butter version of this icing, it can be found in “yellow cake with peanut butter icing”.
1½ cups sugar
½ cup unsweetened cocoa powder
7 tablespoons milk (see Note)
2 tablespoons shortening
2 tablespoons margarine
¼ teaspoon salt
1 teaspoon vanilla extract
Combine the sugar, cocoa powder, milk, shortening, margarine, and salt in a heavy saucepan. Bring to a rolling boil, stirring constantly.
Once it reaches a boil, allow to boil without stirring for 2 minutes, or until it reaches the soft-ball stage. Remove from the heat and add the vanilla. Stir until just slightly cooled. Pour over the cooled cake.
Note: I know you’re going to be tempted not to measure out the milk, but you’re going to have to. Eyeballing it will mess up this icing every time. I know from experience!
enough to frost two 9-inch layers or one 9 x 13-inch sheet cake