Prepare Pastry. Preheat oven to 450 degrees. Combine flour and salt in a large bowl. Cut shortening into flour with a pastry blender until mixture resembles small peas. Push mixture to one side of bowl.
Drizzle 1 tablespoon water along edge of flour mixture. Use a fork to toss some of the flour mixture into water. Move mixture to opposite side of bowl and repeat with 2 more tablespoons of water.
Gather the dough together with your hands. You can add a little more water if the mixture is too dry, but it is best to add as little water as possible. Shape dough into a disk and place on a lightly floured work surface.
Roll dough to 1/8-inch thickness and place in a 9-inch pie plate. Trim excess pastry and fold edges under and crimp. Prick bottom and sides with a fork and bake for 9 minutes, until golden brown. Set pie crust aside to cool and reduce oven to 325 degrees.
Prepare Chocolate Filling. In a medium saucepan, combine sugar, cornstarch, unsweetened cocoa, and salt. Gradually whisk in both milks.
Cook, whisking constantly, over medium-high heat until mixture comes to a boil. Boil for 1 minute whisking continuously.
Remove from heat and gradually whisk a small amount of hot mixture into egg yolks. Continue to add hot mixture to egg yolks while whisking constantly until approximately 1/4 of the mixture has been added to the egg yolks.
Pour egg yolk mixture into the saucepan and whisk well.
Cook mixture, stirring constantly, over medium heat for 3 minutes.
Remove from heat and whisk in butter and vanilla extract. Cover with plastic wrap and keep warm.
Prepare meringue. Using an electric mixer, beat egg whites and cream of tartar at high speed until foamy. Gradually add sugar, 1 tablespoon at a time. Beat until stiff peaks form and sugar dissolves.
Pour chocolate filling into pie crust. Spread meringue over filling, sealing the edges. Bake at 325 degrees for 20 minutes, or until meringue is golden. Cool completely before slicing.
After baking, cool on the counter for 1 hour and then place in refrigerator uncovered for 3 to 6 hours before slicing.