Heat milk and cream in a metal bowl over a saucepan of simmering water, making sure water does not touch the base of the bowl. Make sure the milk does not boil.
In a separate bowl, beat eggs, cocoa, sugar, cornflour, salt, and grated/diced chocolate. Add hot milk one ladleful at a time while whisking, so the chocolate melts but the eggs don't cook.
Pour mixture into a large saucepan and heat very slowly, stirring, until just below boiling point.
Blend in a blender until very smooth, or beat for 5 minutes with a wooden spoon. Transfer to a bowl, cover with cling wrap and chill 2-3 hours in the fridge.