In a medium sauce pan whisk together sugar & cornstarch (do not turn on stove top yet). Whisk in cocoa & salt.
*Make sure you have nothing else to attend to in the next minute becasue this part needs your full attention: Add boiled water and whisk to get out as many clumps as possible. Turn stove to medium-high heat and begin whisking. Cook until mixture thickens (which happens almost instantly). Remove from heat and stir in butter and vanilla. It will be the consistency of pudding.
Spread hot icing onto cooled cake. Enjoy!
*Note: I used an electric kettle to boil my water.