Make and prebake the crust:
Sift the flour, sugar, and salt together in a medium-size to large bowl. Using a pastry blender, a big serving fork, or the tips of your fingers, cut in or pinch or squeeze the shortening until the mixture resembles a bowl of sweet peas. Tossing the mixture quickly and lightly with a fork, sprinkle in the cream or milk 1 tablespoon at a time. (It's better to err on the side of not having enough liquid than to have too much; you don't want a soupy crust.) Continue tossing until the dough holds together when lightly pressed.
With lightly floured hands, loosely gather up the dough into a flat ball, place it in a bowl, and cover with plastic wrap. Refrigerate until you are ready to roll out the crust. I try to chill at least 30 minutes but not too much longer than overnight.
On a lightly floured work surface, roll out the ball of dough into a 12-inch circle about 1/8 inch thick. Place a 9-inch pie plate upside down on top of the rolled-out dough. Using a small knife, cut around the plate, leaving a 1-inch border of dough around the plate. Remove the plate. Fold one side of the crust over in half. Fold the crust into quarters. Pick up the crust so that the center point is positioned in the center of the plate. Unfold the dough and press it firmly into the pie plate.
Trim all excess dough from the edge, except for a 1/2-inch flap of dough around the edge. Tuck the extra flap of dough under itself so it rests on the rim of the pie plate, and then crimp or flute the edge all the way around the pie, and refrigerate the crust for 30 minutes.
Meanwhile, heat the oven to 425°F.
Line the crust with parchment paper and fill it with pie weights or dried beans so the dough doesn't puff up as it bakes. Bake until the top edges start to turn light brown, 8 to 10 minutes. Remove the paper and pie weights, reduce the oven temperature to 350°F, and continue baking until the crust is comepletely golden brown, another 10 minutes. Let it cool completely on a wire rack.
Make the filling:
Beat the yolks until well blended, then set aside. Heat the butter in a medium-size saucepan over low heat until it melts. Add the cocoa, condensed milk, and water and stir constantly with a large spoon or a wire whisk until smooth. Stir in the yolks and the vanilla until smooth and thickened. Remove from the heat. Beat out any lumps that might have formed. Pour the filling into the pie crust.
Make the meringue:
Heat the oven to 350°F. Combine the egg whites, cream of tartar, and vanilla in a medium-size bowl and beat with an electric mixer until soft peaks form. Add the sugar 1 Tbs. at a time and continue beating until shiny, stiff peaks form and all of the sugar has dissolved. Mound in the center of the pie, then spread it evenly around the edge of the inner crust, sealing the meringue to the crust. Use a spoon to create a design of peaks and valleys all over the meringue. Bake until the meringue is golden, 8 to 10 minutes. Let cool completely on a wire rack.