Custard is and always will be a difficult discipline. It can quickly scorch and you need to constantly watch over it. The sous vide technique helps you to spend time on other things in the kitchen while you make your custard.
Remove the vanilla seeds from the pod and mix them with a little sugar.
Put sugar, vanilla, cornstarch, eggs, and milk in a bowl and whisk everything together well.
Put the remaining vanilla pod in a vacuum bag along with the custard mixture. Seal the bag (it does not need to be under vacuum).
Place the bag in your sous vide/water bath, and place a plate over the bag to keep it submerged.
The custard needs 30 minutes at 84 degrees Celsius (183 degrees Fahrenheit). Optionally, take the bag out after 10 minutes and knead the custard a bit.
Do the same again when you take the bag out, to distribute the vanilla in the custard.
Cut a hole in the bag and strain the custard through a sieve. This ensures that any lumps and the vanilla pod are removed.
Refrigerate the custard for about 2-3 hours until it sets.