Add the 1 1/2 cups of the milk to a medium saucepan and heat on medium-low until simmering.

As the milk heats, whisk together the egg yolks, sugar, and salt in a medium heat-proof bowl until light and foamy. Once combined, whisk in the remaining half cup of milk and the cornstarch until smooth and creamy.

When the milk comes to a simmer, whisk the egg mixture continuously while slowly pouring in about 1/3 of the hot milk into it. This tempers the egg to gently heat it so the egg doesn't scramble.

Return the remaining milk to the heat then slowly pour the egg mixture back into the milk while continuously whisking.

Continue to whisk constantly while heating the mixture to a low boil and it reaches 165º F.

Once it reaches 165º, immediately remove from the heat and whisk in the vanilla extract and the butter. Continue to whisk until the butter is completely melted and incorporated.

Place a mesh strainer over a medium glass bowl. Pour the pastry cream through the strainer and whisk to press is through as needed. Leave any clumps in the strainer and discard.

Once all the pastry cream has been strained, smooth out the top of the pastry cream with a spatula. Then place a piece of plastic wrap directly on the pastry cream to prevent a skin from forming when it cools. Refrigerate until cold.

When ready to use, whisk the pastry cream to bring back to a smooth and creamy consistency.