Scald the cream (ie bring almost to boiling point, then remove from the heat) and add the vanilla pod. Leave to infuse until the cream is completely cool. Scrape out the tiny seeds from the pod and leave in the cream.
Over a low heat dissolve the sugar in the water, then turn up the heat and boil rapidly to get a light syrup (it's ready when a little dropped on to a cold plate forms a thread when stretched between finger and thumb. Leave the mixture to cool for just 1 minute.
Place the egg yolks in a basin and begin whisking (ideally with an electric whisk), trickling in the hot syrup as you go. Continue whisking until the mixture is thick and mousse-like. Whisk in the cream. Then pour into an ice cream machine and churn until frozen.