750ml (3 cups) milk
165g (3/4 cup) Chelsea Caster Sugar
2 tablespoons cornflour
3/4 cup good-quality cocoa

Whisk together 1 cup milk with the Chelsea Caster Sugar, cornflour and cocoa.
Place the remaining milk in a saucepan and bring to the boil.
Pour the milk over the chocolate mixture and stir to combine. Return the mixture to the saucepan and stir over low heat for about 10 minutes or until thickened.
Pour the mixture into a shallow container and freeze until frozen at the edges.
Remove from freezer and beat with an electric beater. Pour back into container and re-freeze. Repeat 2 or 3 times. (Alternatively use an ice-cream machine following manufacturer's directions.)