Combine flour, cornstarch, sugar, and salt in a heavy saucepan; gradually stir - in milk. Cook over medium heat, stirring constantly, until thick and bubbly.
Beat egg yolk until thick and lemon colored. Gradually stir in one-fourth of the hot mixture into yolk; add to remaining hot mixture, stirring constantly.
Cook over med urn heat, stirring constantly, about 2 minutes or until thickened. Stir in vanilla and butter; cool. Chill 30 minutes.