Add the milk and vanilla to a saucepan and gently heat until it just starts to steam and lightly simmers. Do not let it boil or develop a skin on top.

In a separate bowl, whisk together the egg yolks, sugar, and cornflour. The mixture will look thick at first, but will loosen and turn pale and smooth as you keep whisking.

Temper the eggs by pouring the hot milk into the egg mixture while whisking constantly.

Pour the combined mixture back into the saucepan and place it over low to medium-low heat. Keep the heat gentle at all times.

Stir continuously using a silicone spatula, making sure to scrape the bottom and sides of the pan. After a minute or two, the custard will begin to thicken.

Once it starts to set, switch to a whisk and whisk continuously until the custard becomes smooth and thick. Cook for about 1 minute, or until it coats the back of a spoon.

Remove from the heat and whisk in the butter until fully melted and incorporated.

Transfer the custard to a bowl and cover tightly with cling film, pressing it directly onto the surface. Chill in the fridge for 2-3 hours before using.

Give the custard a quick whisk to loosen it and make it smooth before using.