Take 50ml/2fl oz of the measured milk and combine with the cocoa powder to form a thin paste. Heat the remaining milk just to the boiling point, then remove from the heat and allow to cool slightly.
In a large heat-proof bowl, whisk the egg yolks with the sugar until very pale and thick (this is best done with an electric whisk).
In a thin stream, add the hot milk to the egg mixture, whisking continuously, until completely combined. Add the cocoa paste and return the mixture to the saucepan.
Over a medium heat, stirring constantly, cook the custard very gently for at least five minutes. This is important to cook the cocoa powder. Continue to cook the custard very gently until it thickens and coats the back of a spoon (if you draw a line through the custard on the back of the spoon with your finger, it will stay clear).
Remove from the heat and pour the mixture into a large bowl. Stir in the vanilla extract and the chopped chocolate until melted.
Allow the mixture to cool , then refrigerate, covered, for at least eight hours, preferably overnight.
Churn in an ice cream maker according to the manufacturer’s instructions. Alternatively pour the mixture into a large plastic box. Cover and freeze for an hour. Remove from the freezer and whisk with a hand-held electric mixer or stick blender until the ice crystals are broken up. Repeat three more times and then freeze until firm.