Place the sugar, cornflour and cocoa powder in a heatproof bowl and whisk in the eggs. Bring the milk to the boil in a saucepan, then whisk a little of it into the egg mixture. Transfer the chocolate mixture to the saucepan, stir well and cook for 3–5 minutes, stirring continuously, until thickened.
Place the chopped chocolate in the bowl, sieve the chocolate custard on top and stir until smooth. Cover the surface with clingfilm to prevent a skin forming, then place in the freezer for 15 minutes, stirring occasionally, until cool.
When the chocolate custard is cool, divide between glass bowls, top with whipped cream and sprinkle with grated chocolate.