Mix sugar, cocoa powder, and milk together in a large saucepan.
Cook over medium-high heat until large bubbles form and reaches a hard boil. Boil one minute at a hard boil. Remove from heat and add butter and vanilla.
Beat with a wooden spoon until thick and smooth.
Pour over cake. It will harden as it cools.
Notes
This icing hardens quickly. I prefer to use it as a poured icing as photographed or poured onto a sheet cake. I would not recommend using it as a spreadable frosting on a layered cake.