In medium saucepan, combine sugars, cornstarch, and salt; mix well.

Gradually stir in milk, mixing until smooth.

Over medium heat, bring to boiling, stirring constantly.

Boil one minute, stirring constantly. Remove from heat.

Stir half of mixture into yolks, mixing well; pour back into saucepan.

Bring back to boil; boil one minute longer. Remove from heat. Stir in butter and vanilla.

Pour immediately into pie shell. Refrigerate until ready to serve.

Serve topped with whipped cream.