1. In a medium bowl, beat egg yolks and sugar until thick and light; beat in flour and cornstarch.

2. In a saucepan, bring milk just to a boil; whisking constantly, gradually add milk to egg mixture.

3. Return mixture to saucepan. Bring to a boil, whisking constantly to prevent scorching, until mixture is thick and pulling away from sides of pan, 4 to 5 minutes. Stir in vanilla.

4. Using a rubber spatula, press mixture through a sieve into a bowl. Cover surface directly with plastic wrap, and cool to room temperature in refrigerator.