1. Heat milk to simmer in a small pot. In a medium bowl, whisk together sugar and cornstarch. Add egg and yolk; whisk to blend.
2. Slowly pour hot milk into egg mixture, whisking constantly. Return milk mixture to saucepan. Whisking constantly, heat over medium until sauce thickens and boils (about 5 minutes). Pour into another bowl to stop cooking. Chill, stirring occasionally (about 4 hours).