Prepare Baked Pastry Shell. Cool. Meanwhile, for filling, in a large saucepan, combine the sugar, cornstarch, and salt. Gradually stir in milk. Cook and stir over medium-high heat until thickened and bubbly. Cook and stir for 2 minutes more. Remove from heat.
In a medium bowl, slightly beat the egg yolks with a whisk or fork. Gradually stir 1 cup of the hot mixture into the yolks. Return egg yolk mixture to the hot filling in pan. Bring to a gentle boil. Cook for 2 minutes more. Remove from heat; stir in vanilla.
Place semisweet chocolate pieces in a bowl. Pour 2 cups of the filling over chocolate. Stir until chocolate is melted and well mixed. Spread chocolate mixture evenly over bottom of baked pie shell. Carefully spoon the vanilla filling over the chocolate layer. Cover and chill until serving time.
Top with whipped topping or cream, if you like. Makes 10 servings.
In a large bowl, stir together flour and salt. Using a pastry blender, cut in shortening until pieces are peasize. Sprinkle 1 tablespoon cold water over part of the mixture; gently toss with a fork. Push moistened dough to the side of the bowl. Repeat moistening dough, using 1 tablespoon cold water at a time, until all the dough is moistened. Form dough into a ball. On a lightly floured surface, use your hands to slightly flatten dough. Roll dough from center to edge into a circle about 12 inches in diameter. To transfer pastry, wrap it around the rolling pin. Unroll pastry into a 9-inch pie plate. Ease pastry into pie plate, being careful not to stretch pastry. Trim pastry to 1/2 inch beyond edge of pie plate. Fold under extra pastry. Crimp edge as desired. Generously prick bottom and side of pastry in pie plate with a fork. Prick all around where bottom and side meet. Line pastry with a double thickness of foil. Bake in a 450 degree F oven for 8 minutes. Remove foil. Bake for 5 to 6 minutes more or until golden. Cool on a wire rack.