In a large saucepan, whisk together milk, 1⁄2 cup (100 grams) sugar, and vanilla. Heat over medium heat until steaming. (Do not boil.)

In a large bowl, whisk together egg yolks, cornstarch, salt, and remaining 1⁄2 cup (100 grams) sugar. Gradually add warm milk mixture, whisking constantly. Pour egg yolk mixture into saucepan, and cook over medium heat, whisking constantly, until thickened and boiling, 4 to 5 minutes.

Strain mixture through a fine-mesh sieve into a large bowl. Stir in butter in two additions. Cover with plastic wrap, pressing wrap directly onto surface of pastry cream to prevent a skin from forming. Refrigerate until cold, about 4 hours, or overnight.