In a medium saucepan, whisk together the sugar, flour, cornstarch, and salt until well combined.
In a separate bowl, whisk together the egg, yolks and milk until well combined, I do this by whisking the egg and yolks together first, then slowly whisking in the milk.
Pour the egg mixture into the sugar mixture and mix well until there are no lumps.I do this by slowly adding the liquid mixture a little at a time and whisking well between each addition.
Turn on the heat to the stove on medium and continue to whisk the mixture.
Whisk the mixture continuously as it cooks and begins to thicken. The creme patissiere is finished cooking once it is noticeably thicker and when you dip a spoon into the mixture and run a finger down the spoon, it leaves a line in the pastry cream.
Turn off the heat and remove the pan from the stove. Stir in the butter and the vanilla. Mix until the butter is melted and thoroughly mixed in.
Strain the creme patissiere through a fine mesh strainer into your container.
Place plastic wrap directly on the creme patissiere so that it is directly touching. This is to prevent skin forming on the pastry cream. Chill until completely cooled before using.