In a small saucepan, bring the milk to a boil over medium heat. Meanwhile, whisk the egg yolks and sugar together in a small bowl. Add the cornstarch to the egg-yolk mixture and mix well, until smooth and free of lumps.
When the milk boils, add it to the egg-yolk mixture and stir well. Return the mixture to the saucepan and whisk rapidly over high heat, whiskin