Combine sugar, cornstarch, and salt in a heavy saucepan; stir well. Combine egg yolks and milk; gradually stir into sugar mixture. Cook over medium heat, stirring constantly, until mixture thickens and boils. Boil 1 minute, stirring constantly. Remove from heat; stir in butter and vanilla. Immediately pour into pastry shell. Cover filling with wax paper. Let cool 30 minutes; then chill until firm. Beat whipping cream until foamy; gradually add powdered sugar, beating until soft peaks form. Spread whipped cream over