In a bowl or in the upper part of double boiler, off the heat, whisk together the egg yolks, sugar and cocoa powder until the mixture lightens. Whisk in the milk and cream.
Cook in the double boiler, stirring with a wooden spoon, for about 15 minutes or until it thickens and a candy thermometer reads 80 °C (175 °F), no higher . Cover and refrigerate to chill completely, about 3 hours.
Add the chocolate.
If you work with an ice cream machine, process the mixture according to the manufacturer's instructions. Freeze (about 45 minutes).
If you do not have an ice cream maker, pour the mixture into a large shallow dish and freeze for 2 hours. Stir and freeze again for 1 hour. Process the ice cream in a food processor and return to the freezer until ready to serve.