In a saucepan, combine sugar, cornstarch and salt. Mix well. Gradually stir in milk.
Cook over medium heat, stirring constantly until the mixture thickens and boils. Boil 1 minute. Remove from heat.
Place egg yolks in a medium mixing bowl and gradually stir in at least half of the hot milk mixture. Then blend the egg yolk mixture back into the saucepan. Boil 1 minute more, stirring constantly. Remove from heat.
Blend in butter and vanilla. Cool, stirring occasionally.
Pour into baked pie shell and chill thoroughly, at least 2 hours.
Top with stabilized, sweetened whipped cream.