Mix sugar, cocoa, butter and milk together. Over medium heat, bring to a boil and boil for 1 minute. Add vanilla. Cool partially, then beat with a mixer for 3 minutes or until spreadable consistency.
This is not a fluffy kind of frosting that you use to decorate a cake. I did use it one the layer cake in the picture, but this is more of a icing that would go on a sheet cake. The mixture will harden some, but it definitely shouldn't be soupy. Be careful not to heat it over too high a heat, or the sugar will start to turn to candy, which will make the mixture grainy. Spread on the cake quickly and let it set until completely cooled.