Beat egg yolks and sugar until thick and pale.
Place cocoa powder into a saucepan, add milk and cream slowly while heating over a medium heat. Stir constantly until mixture is almost boiling.
Add Bar ones to the milk mixture, remove from heat.
Whisk milk mixture into the egg while pouring a steady stream of milk into the egg.
Set aside to cool, or cool over a bowl of ice.
If you have an ice-cream machine, place the cooled ice-cream in the machine and churn until thick.
Pour mixture into a wide container and freeze for at least 6 hours. If you don’t have an ice-cream machine, freeze for 30 minutes and then beat well and then return to the freezer.