In another pan bring cream and all but 1/4 cup of milk to scalding point. Stir half scalded mixture into melted chocolate and mix well, then pour back into the saucepan and set over low heat. Mix in the reserved 1/4 cup milk with the cocoa, sugar and cornflour to make a smooth paste. Stir into the chocolate mixture and whisk or stir constantly until it comes to the boil and thickens slightly.
Add vanilla, if desired and beat briskly with wire beater or hand rotary beater until mixture is foamy.
Pour into cups and serve immediately topped with whipped cream.