Add the plant milk to a saucepan along with the vanilla pod (cut in half) and let it simmer over a low heat for 10 minutes allowing the vanilla to infuse the milk.
(While keeping an eye on the milk to ensure it doesn't scorch) add the sugar and cornflour to a mixing bowl and whisk together to ensure they are well combined and their are no lumps.
Carefully pour a quarter of the milk into the sugar and cornflour mixture. Whisk well until it has formed a lump free paste.
Slowly pour the rest of milk into the paste while whisking well to ensure you have a smooth, thick custard.
Transfer the custard back into the pan and cook on a medium heat for 2-3 minutes, stirring constantly to ensure it doesn't scorch or create lumps.
The custard is ready when there are large bubbles of steam.
Take the custard off of the heat and beat in the butter until smooth.
Spoon the custard into a clean bowl, cover with clingfilm so that the clingfilm is in direct contact with the custard. (See note 3).
Leave the custard to cool then place into a fridge for at least an hour or until the custard has set.