• 2 cups milk
• 2 tablespoons cornstarch
• 1/3 cup white sugar
• 1 large egg
• 2 egg yolks
• 1/4 cup unsalted butter, cubed
• 1 teaspoon vanilla extract

Whisk 1/3 of the cold milk and cornstarch together in a medium bowl until smooth. Pour remaining milk and sugar into a small saucepan. Heat over medium heat, stirring occasionally, until milk begins to simmer; remove from heat and set aside. Stir egg and egg yolks into cornstarch mixture; gradually whisk in hot milk. Pour mixture back into the small saucepan; cook over medium-low heat, whisking constantly, until thickened and mixture coats the back of a spoon, about 3 to 5 minutes. Remove custard from heat; whisk in butter and vanilla until smooth. Transfer into a bowl and cover directly with plastic wrap to prevent a skin forming; allow to cool before using.