Put the milk, cream and chocolate in a medium pan and bring to a simmer. Mix until combined, then take off the heat and allow to cool slightly.
Meanwhile, whisk together the sugar, 2 whole eggs and 2 egg yolks in a bowl until pale and creamy.
Pour the chocolate and milk mixture on to the egg mixture and whisk together to combine. Transfer the mixture to a clean pan. Cook gently over a low heat for 5 minutes until the mixture is thickened.
Strain through a sieve into a clean, freezer-safe bowl and mix in the vanilla extract. Cover and leave until cold. Freeze for 6 hours, beating through with a wooden spoon for 1 minute every 2 hours.