In a large saucepan, heat milk and ½ cup (100 grams) sugar over medium heat, whisking constantly, until steaming. (Do not boil.)

In a large bowl, whisk together egg yolks, cornstarch, salt, and remaining ½ cup (100 grams) sugar. Gradually add hot milk mixture to egg yolk mixture, whisking constantly. Return mixture to saucepan; cook over medium heat, whisking constantly, until thickened and bubbly, 4 to 5 minutes. Whisk in vanilla.

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

In a chilled large bowl, using a chilled balloon whisk, whisk cold cream until stiff peaks form.

Whisk cold pastry cream until smooth. Fold into whipped cream in three additions. Use immediately.