Put the milk in a saucepan and break the plain chocolate into it. Heat gently over a low heat until the chocolate melts, then cook over a high heat until almost boiling.

Whisk the egg yolks and sugar together until pale and thick. Gradually pour on the chocolate milk, stirring. Return to the saucepan and cook over a gentle heat, stirring continuously, until it coats the back of a spoon. Do not boil. This takes about 20 minutes.

Pour into a shallow freezer container and leave to cool. When the mixture is cool, cover and freeze for 2 hours, until mushy.

Turn into a large bowl and beat with a fork or whisk to break down any large ice crystals. Whip the cream until it just holds its shape, then fold into the frozen custard with the chocolate chips. Return the mixture to the freezer container and freeze for 2 hours.

Beat the mixture again, to break down any large ice crystals, then freeze until firm.

Transfer to the refrigerator to soften for 30 minutes before serving.