Meanwhile, in heavy 2-quart heavy saucepan, stir together sugar and cornstarch; stir in milk. Cook and stir over medium heat until thickened and bubbly. In medium bowl, beat eggs with wire whisk just until blended. Gradually stir 1 cup hot milk mixture into egg mixture, whisking constantly, until combined. Pour milk and egg mixture back into saucepan. Return to medium heat. Cook, stirring constantly, 1 to 2 minutes longer, or until thick and bubbly. Remove from heat. Stir in butter and vanilla. Press plastic wrap on filling to prevent a tough layer from forming on top. Refrigerate at least 1 hour.