Combine the cornstarch with the sugar in a mixing bowl. In a separate bowl, whisk together the egg yolks. Pour 1/2 cup of the milk into the cornstarch mixture and whisk together. Whisk the egg yolks into the cornstarch mixture until smooth.

In a small sauce pan over medium heat, bring the remaining 1 ½ cups milk to a simmer (be sure not to boil) Once it comes to a simmer, remove the pan from the heat.

Drizzle the hot milk mixture a little at a time into the egg mixture. Keep on whisking while you drizzle in the milk.

Pour the mixture back into saucepan and cook over medium-low heat. Whisk vigorously until it comes to a boil and the whisk leaves a trail in the pastry cream (about 5-6 minutes).

As soon as the cream has the right consistency, take the pan off the heat. Stir in the vanilla and he butter.

Place plastic wrap over the pastry cream surface and let it cool completely in the refrigerator.