In a large saucepan, whisk together milk, 1⁄4 cup (50 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 1⁄4 cup (50 grams) sugar. Gradually add warm milk mixture, whisking constantly. Return mixture to saucepan, and cook over medium heat, whisking constantly, until thickened and boiling, 4 to 5 minutes.

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