Chocolate Cream Pie

1 cup sugar
½ cup cornstarch
3/8 cup cocoa
1/8+ tsp salt
4 egg yolks, beaten
4 cups milk
2 T butter
2 tsp vanilla
¾ whipping cream
1/3 cup sifted powered sugar

Combine sugar, cornstarch, cocoa and salt and in a heavy saucepan; stir well.  Combine egg yolks and milk; gradually stir into sugar mixture.  Cook over medium heat, stir 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 plastic wrap.  Let cool 30 minutes; then chill until firm.