Homemade Vanilla Custard

I haven't tried this one yet, but my parents make it frequently and they say it's the best custard recipe they've found.

Homemade Vanilla Custard

1/2 c. sugar
3 TB cornstarch
1/2 teaspoon salt
4 egg yolks
3 cups milk
2 TB butter or margarine
2 tsp vanilla extract

Bring first 5 ingredients to a boil in a heavy sauce pan over medium heat (about 20 minutes), whisking constantly; boil, whisking constantly or until thickened. Remove from heat. Stir in butter and vanilla. Place heavy duty plastic directly on surface of custard to keep "skin" from forming. Chill 2 hours.