Bring the milk and vanilla to hot and steamy in a saucepan. Keep an eye on the pan as the milk can boil over very quickly!
Mix the sugar, egg yolks, corn starch and sugar together until thoroughly incorporated.
Pour half of the warmed milk over the egg mixture and whisk vigorously until smooth and thoroughly combined
Pour the egg mixture into the saucepan with the milk and continue to whisk over a medium heat
Cook until the mixture thickens, being careful not to let it burn on the bottom of the pan. The mixture will go very lumpy – don't worry, this is supposed to happen! Just keep whisking and the mixture will go smooth, thickened and glossy. Cook out for another two minutes, then remove from the heat
Add unsalted butter and mix well
Empty your crème pâtissière into a bowl and close cover with cling film to prevent a skin forming. Allow to cool, then place in the fridge until needed