In a small bowl, whisk together sugar, egg, egg yolks, and cornstarch. While continuing to whisk, slowly add about 1/4 cup of the heated milk. Repeat with about 1/2 cup more milk, until the eggs are tempered. Slowly pour the eggs into the saucepan of milk, continuously whisking. Continue to stir the mixture over medium heat until it thickens. Remove from heat and transfer to a medium bowl. Cover the top with plastic wrap, making sure it is touching to prevent a film from forming, and refrigerate until chilled and ready to serve.