For crust, in a large mixing bowl stir together flour, the 2 tablespoons cocoa, and salt. Using a pastry blender, cut in shortening and 1/3 cup butter until pieces are pea-size. Sprinkle 1 tablespoon of water over the mixture; gently toss with a fork. Push moistened dough to the side of the bowl. Repeat, using 1 tablespoon of water at a time, until all the dough is moistened.
Preheat oven to 450 degrees F. Divide dough in half. Form each half into a ball. Cover and refrigerate one ball. On a lightly floured surface, flatten remaining ball. Roll from center to edges into a 12-inch circle. Transfer to a 9-inch pie plate; avoid stretching. Gently press into plate. Trim edges. Line with double-layer of foil.
Bake pastry for 8 minutes. Remove foil; bake 5 minutes more or until firm and dry. Sprinkle with the semisweet chocolate chips. Cool completely on a wire rack. Reduce oven temperature to 325 degrees F.
For filling, in a 2-quart saucepan stir together sugar and cornstarch. Stir in milk and unsweetened chocolate. Cook and stir over medium heat until thickened and bubbly. Cook and stir 2 minutes more.
Slowly stir 1 cup of the hot mixture into the beaten egg yolks. Pour egg yolk mixture into hot filling in pan. Bring just to boiling; reduce heat. Cook and stir for 2 minutes. Remove from heat. Stir in butter and vanilla. Let cool 10 minutes.
Meanwhile, on a lightly floured surface, flatten chilled dough. Roll from center to edges to form a 12-inch circle. Pour warm filling mixture into baked pastry shell. Place pastry top over the filling mixture, pressing into pastry edge to seal. Trim dough to edge of pie plate.
Gently prick top of pastry with a fork. Bake for 25 minutes or until crust is firm and dry. Cool for 1 hour on a wire rack. Cover and chill for 2 to 24 hours before serving.
To serve, invert a serving platter on top of pie. Turn pie upside-down. Remove pie plate. If desired, top with whipped cream and grated chocolate.