To a large sauce pot over medium heat, add the milk
Scrape the seeds from one vanilla bean and add to the milk.
Heat until it starts to steam but before it starts to simmer and form bubbles.
Remove from heat and set aside to steep.
Meanwhile, in a large bowl combine the sugar, cornstarch and egg yolks.
Whisk until it is light and creamy.
In a new large bowl, add the cubes of butter. Place a strainer overtop of this bowl, set aside.
Now the milk should be ready. Gently, and gradually, pour a little bit of the hot milk into the egg and sugar mixture, whisking constantly.
Pour the egg and milk mixture back into the sauce pot and return it to medium heat. Stirring until it is glossy and thickened.
As soon as you see a few bubbles form, from it beginning to simmer, remove it from the heat and pour it through the strainer, onto the cold butter.
Remove the strainer, and stir everything together.
Place a piece of cling-film on top of the pastry cream. Allow to cool completely before using. The cling-film will stop it from forming a skin.