Put the milk into a pan and bring to the boil. Beat the egg yolks with the vanilla seeds until foamy, gradually trickling in the sugar. Pour the hot milk on to the egg yolk mixture, stirring constantly with a whisk.
Return the mixture to the pan and reheat, stirring, until it thickens slightly. Then remove from the heat. Do not let it boil! Melt the chocolate and 3 tbsp cream in a bain-marie, stirring, then mix into the egg and milk mixture with the cocoa.
Cool in the refrigerator for about 1 hour. Lightly whip the rest of the cream and fold into the chocolate milk mixture. Transfer to an ice cream maker and freeze for about 30 minutes.
Alternatively transfer to a metal dish and put into the freezer for about 4 hours, stirring vigorously every hour. Scoop out with an ice cream scoop, pile a few scoops in each dish and serve immediately.