Combine the cornstarch with the sugar in a mixing bowl.
Lightly beat 4 egg yolks (in a seperate bowl)
Pour 1/2 cup of the milk into the sugar and cornstarch mixture and stir well with a whisk.
Blend the lightly beaten yolks into the cornstarch and sugar mixture
Stir with a wooden spoon until it is completely smooth
In a sauce pan or pot, combine the remaining 1 ½ cups milk. Bring to a simmer (do not boil) and remove the pan from the heat
To temper, you can gradually add about one-third of the milk mixture at the time, but I like to drizzle the hot milk mixture into the egg mixture little by litte while I’m whisking, just in case. Don’t forget to whisk constantly!
Return the mixture to the saucepan and continue cooking over low-medium heat, vigorously stir with a whisk until the mixture comes to a boil and the whisk leaves a trail in the pastry cream. It will take a little while to come to a boil – about 5 minutes
Remove the pan from the heat as soon as you reach that consistency (if you leave on a stove too long, it won’t be soft and creamy pasty cream), and add the vanilla extract and the butter. Stir well until they are well blended.
Cover the pastry cream surface with plastic wrap and put into the fridge until the pastry cream is cool which takes about 15 minutes.