I just found a recipe for the frosting that may be different. It is:

CHOCOLATE FROSTING FOR SMITH ISLAND CAKE

2 cups sugar
1 cup evaporated milk
5 ounces unsweetened chocolate, chopped
1 stick (1/2 cup) unsalted butter
1/2 tsp to 1 tsp vanilla

Combine sugar and evaporated milk in a medium saucepan over medium - low heat. Add the chocolate and butter; warm through, stirring, until both have melted. Increase the heat to medium and cook, stirring occasionally, for 10 - 15 minutes. Remove from the heat and add the vanilla extract, stirring to combine. The icing will be thin but will thicken as it cools. (Or so the recipe states)

Hope this helps!