Place a container in the freezer. Split the vanilla pod lengthways, scrape the seeds out with the point of the knife and tip into a pan with the cream and pod. Bring to the boil, then remove from the heat and leave to infuse for at least 20 mins. For the best flavour, this can be done a few hours beforehand and left to go cold (see tips, below).
In a large bowl, whisk the sugar and egg yolks together for a few minutes until they turn pale and fluffy. Put the vanilla cream back on the heat until it’s just about to boil, then carefully sieve the liquid onto the yolks, beating with the whisk until completely mixed.
At this point, get a large bowl of iced water and sit a smaller bowl in it. Pour the custard back into the pan and cook on the lowest heat, stirring slowly and continuously, making sure the spoon touches the bottom of the pan, for about 10 mins until thickened (see rules, below). Strain the custard into the bowl sitting in the iced water and leave to cool, then churn until scoopable (see methods, below).
Roll pieces of cookie dough into balls or thin snakes, flatten them with your hands and layer through the ice cream. Freeze until solid.