Put the chocolate in a plastic bag, seal and freeze for at least 30 mins. Bash the frozen chocolate with a rolling pin until it shatters into small chunks inside the bag.
Pour the lactose free milk and cream into a medium heavy-based pan, then add the vanilla pod, scraping the seeds into the mixture.
Heat the creamy milk, stirring occasionally, until almost boiling. Add the mint leaves and stalks, cover with a lid, and leave for 15 mins.
Strain the liquid, pressing the mint with a spatula to extract maximum flavour. Discard the leaves and stalks, and scoop out the vanilla pod.
Whisk the egg yolks and caster sugar until thick and pale – about 5 mins if you are using an electric whisk. Return the minty cream to a clean pan and bring back to almost boiling.
Pour half the hot liquid onto the egg and sugar mixture, whisking to combine everything before adding the remaining liquid.
Return the custard to the rinsed-out pan and cook over a low heat, stirring all the time, until lightly thickened – about 3-4 mins.
Take care it doesn’t get too hot or it may curdle. Strain the custard into a bowl and leave to cool, stirring occasionally to prevent a skin forming. Chill the custard until really cold – it’s a good idea to leave it overnight.
Churn the custard in an ice-cream machine until frozen. Add the chocolate pieces just before it sets. If you don’t have a machine, pour the custard into a chilled container and freeze for 2-3 hrs until the sides become icy.
Whisk thoroughly to break up any crystals, then add the chocolate and freeze until solid. Leave to soften in the fridge for 20-30 mins, then serve in bowl