Freeze the canister from the ice-cream machine a day before you make the ice cream (if your machine requires you to). Next day, mix the sugar, custard powder and cornflour with 2 tbsp milk to make a thin paste. Beat in egg yolks.
Pour the rest of the milk into a pan, scrape in the vanilla seeds, add the pod, then bring to the boil.
Pour this slowly over the cornflour mix, stirring all the time as you do so. Clean the pan, then pour the milk mixture and vanilla pod back into it. Cook over a medium heat, stirring all the time, until it just comes to the boil and is thick enough to coat the back of a spoon.
Remove from the heat, stir in the crme frache, then pour into a bowl.
Place a piece of greaseproof paper over the surface to prevent a skin forming, then leave to cool. Put into the fridge until really cold for at least 4-5 hrs (preferably overnight).
Remove the paper and vanilla pod from the custard, then transfer the custard into a jug. Turn on the ice cream machine, then slowly pour in the custard. Leave to churn for 10-30 mins (depending on your machine). When it stops, spoon into a plastic container, cover with cling film and a lid, then freeze for at least 3-4 hrs. (Will keep for up to 1 month. Do not re-freeze.) For the best taste, remove from the freezer and soften in the fridge for 1-1 hrs before serving.
