Mix sugar, cocoa powder and cornflour in a microwave-safe bowl. Gradually whisk in the milk until well combined.
Place in the microwave for 3 minutes. Stir, then cook at 1 minute intervals, stirring, for the next 2 to 4 minutes, or until shiny and thick. Or put on a double boiler while constantly stirring. Whisk until shiny and thick.
Stir in the vanilla. If you want, you can eat it hot or put it in the fridge to cool for 4 hours. Enjoy!
You can add a bit of butter and a dash of salt to enhance the flavour.