This icing is made with granulated suagar instead of confectioners. Back in the day when my husband and I were in school, the cafeteria served a chocolate cake with "hard" icing. You could literally break it off and eat it like (fudge) candy. It still had a soft texture though. My husband is always after me to make it. I've searched and searched and this recipe comes closest to what I remember. I found this one on Food.com but tweaked the directions to ensure that the icing sets right and added 1 tsp. vanilla as an option. Original recipe calls for margarine, I haven't tested it with butter.
cocoa powder, unsweetened
margarine or butter
vanilla extract, optional
1Put all ingredients (except vanilla) in a heavy large saucepan.
2Bring to a full boil over medium heat, stirring constantly. Scrape sugar crystals off the sides of the pan.
3Continue to cook (rolling boil) for 8 minutes; stirring constantly.
4Remove from heat and stir in vanilla, if desired. Allow to cool 5 minutes.
5Beat with spoon or mixer until frosting begins to thicken but is still pourable. Spread on cake. Work fast because icing begins to harden quickly. Allow to cool before serving.