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, in a bowl, whisk together the sugar, eggs and egg yolks 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 until the mixture is thickened (about 5 minutes).
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. Serve with a mint sprig if you prefer.