Cake frosting 5/10
125 g (about 3/4 cup) butter
1 c milk
1 c sugar
1/2 c cocoa powder

mix butter sugar milk, and cocoa on the stove
bring to a boil
boil 10 minutes stirring constantly
cool
frosting will be very thin and glossy

Source:  I made this.

Notes:  This was done in a pinch.  It turned out ok but it wasn't excellent.