Prepare piecrust; cool. In 2-quart saucepan with spoon, mix together sugar, flour and salt; stir in milk.
Stir in chocolate and over low heat, cook mixture, stirring constantly, until chocolate is completely melted.
With wire whisk, beat until chocolate is blended; increase heat to medium: cook, stirring till mixture is thickened and boils (about 10 minutes); remove at once from heat.
In cup with wire whisk, beat egg yolks with small amount of hot chocolate mixture.
Slowly pour egg mixture into saucepan, stirring rapidly. Over low heat, cook, stirring, until very thick (do not boil) and mixture mounds when dropped from spoon.
Remove from heat; stir in butter and vanilla then pour into piecrust. Cover surface with plastic wrap to prevent skin forming. Refrigerate until set, about 4 hours.
Just before serving: In small bowl with mixer at medium speed, beat cream until stiff peaks form. Discard plastic wrap on filling. Spread whipped cream on top of pie.