Grandma & mom used to make this and it has the dark chocolate flavor so many love these days. It is a beautiful, shiny, not too sweet icing. I use it on my super-moist scratch chocolate cake. Do any of you remember it? Google it, you'll be surprised.
Have water at boiling. On stovetop mix sugar, cornstarch, grated chocolate & salt with wire whisk. Heat burner and pour boiling water into dry mixture. Set over heat and whisk until bubbling and thickened. Remove from heat (1-2 minutes)add butter and vanilla and whisk. Have cooled cake ready and pour and spread frosting right away. You may use a spatula to form pretty peaks. It should be shiny. Not a sweet icing.