Make the crust:
In a large bowl, stir together the flour, sugar and salt. Cut in the shortening with a pastry blender or 2 knives used like a scissor, or mix with your fingertips until the mixture resembles coarse crumbs. Sprinkle 2 tablespoons of the ice water over the flour mixture, tossing with a fork; adjust enough of the remaining water, by the tablespoon, tossing, so that the dough holds together. Flatten into a disk and wrap in plastic wrap. Refrigerate for 1 hour.
Preheat the oven to 425 degrees F. On a lightly floured surface, with a lightly floured rolling pin, roll out the dough into an 11-inch circle. Fit into a 9-inch pie plate. Fold the edge under to make a stand-up edge; crimp. Prick the bottom all over with a fork. Bake until golden brown, 12 to 14 minutes. Remove the pie plate to a wire rack to cool.
Make the filling:
In a heatproof bowl, lightly beat the egg yolks to break up, and then set aside. In a small, heavy saucepan, stir together the sugar, flour and salt. Gradually stir in the scalded milk. Cook over medium-low heat, whisking, until thickened and bubbly, 6 to 8 minutes. Stir a little of the hot mixture into the yolks. Stir the yolk mixture back into the saucepan. Heat over very low heat, stirring, for 2 minutes, and do not let boil. Remove the saucepan from the heat. Whisk in the chocolate, butter, and vanilla until melted and smooth. Scrape into a clean bowl. Press a piece of plastic wrap directly on the surface of the filling and let cool for 10 minutes. Remove the plastic wrap and scrape the filling into the piecrust. Cool on a wire rack.
Make the meringue:
Preheat the oven to 350 degrees F.
In a small saucepan, stir together the cornstarch, 2 tablespoons of the sugar, and the water. Cook over medium heat, stirring occasionally, until the mixture comes to a simmer and thickens, 2 to 3 minutes. Remove the saucepan from the heat. In a large bowl, add the salt to the egg whites. With an electric mixer at medium speed, beat just until soft peaks form. Add the cornstarch mixture and beat until creamy. Gradually beat in the remaining sugar and continue to beat until soft peaks form, 6 to 8 minutes. Spread the meringue mixture over the cooled chocolate filling, make sure the meringue touches the piecrust edge all the way around. Bake until the meringue is golden and its internal temperature registers 150 degrees F on an instant-read thermometer, 25 to 30 minutes. Serve soon after. Cover and refrigerate any leftovers.