Take a Microwave safe bowl, whisk together the sugar, cocoa, and cornflour. Add milk slowly and combine well.
Microwave for 3 minutes in an interval of a minute. Remove and whisk stiffly. Then again cook for 2 minutes, at 1 minute intervals. Do this two times. Whisk until shiny and thick.
Using a spatula, gather everything and even out. Cover with cling wrap and chill until set, which might take about 2 to 3 hrs. Or if you want to serve right away, freeze for 10 mins.