In a heavy saucepan combine sugar, cornstarch.

Stir in milk.

Cook and stir over medium heat until thickened and bubbly.

Cook and stir for two minutes more.

Lightly beat egg yolks in a medium bowl.

Gradually stir in one cup of the milk mixture into the egg yolks. (I did this about ½ tablespoon to 1 tablespoon at a time, and took several minutes)

Add the egg yolk mixture to the milk mixture in saucepan.

Bring to a gently boil; reduce heat.

Cook and stir for two minutes more.

Remove from heat.

Stir in butter and vanilla.

Pour pudding into a heatproof bowl.

Cover with plastic wrap.

Chill and do not stir during this time.