This recipe is from the 1920s or 1930s. It may be much older. The results are better than any commercial frosting found in a box or container.
I recommend using a candy thermometer unless you really want to see how cooks did things in the past. I still use the drop of syrup in the cold water method. This icing is very good on either yellow or chocolate cakes.
• 2 cups of granulated sugar
• 1/8 teaspoon of cream of tarter
• 2 squares of chocolate
• 3/4 cup milk
• 2 tablespoons of butter
• 1 teaspoon of vanilla
Heat the sugar, cream of tartar, chocolate and milk in a medium sauce pan. Stir until the sugar dissolves, then do not stir again. Cook over low heat until 230 degrees on a candy thermometer or the syrup forms a soft ball in cold water. Remove the pan from the heat, add the butter and vanilla without stiring in. Cool until lukewarm then beat until right to spread on a cake.
Instead of the baking chocolate you can use 6 tablespoons of cocoa plus 2 tablespoons of butter.
The old fashioned way of testing was to take a bit of it on a spoon and let it sheet off into a standard sized glass of cold water. It will harden slightly in the cold water and form into a ball if about right. Take the tiny ball of candy out of the glass (about the size of a pea or slightly larger) and if it stays together in a ball and is still soft it is right.