Step 1

Whisk egg yolks, 2 tablespoons sugar, and cornstarch in medium bowl until mixture is pale yellow and thick, about 1 minute; set aside. Heat milk, salt, and remaining 6 tablespoons sugar in medium saucepan over medium heat until simmering, stirring occasionally to dissolve sugar.

Step 2

Gradually whisk half of milk mixture into yolk mixture to temper. Return milk-yolk mixture to saucepan. Return to simmer over medium heat and cook, whisking constantly, until mixture is thickened and 3 or 4 bubbles burst on surface, about 1 minute. Off heat, whisk in butter and vanilla. Transfer mixture to clean bowl, press parchment paper directly onto surface, and refrigerate until set, at least 3 hours.