For the pie crust: In a medium bowl using a pastry blender or fork, mix together the flour, butter, shortening, and salt until the mixture resembles coarse crumbs. Add 2 tablespoons ice water and mix just until the water is incorporated. Add more water as needed to make a smooth dough. Form the dough into a disk, wrap tightly in plastic wrap, and let rest in the refrigerator for 1 hour.

Remove from the refrigerator and place on a lightly floured surface. Roll the pastry to a large round, about 12-inches in diameter. Transfer to a deep dish 9-inch pie pan, trim to within 1/2-inch of the pan, and crimp decoratively. Prick with a fork, cover, and refrigerate for 20 to 30 minutes.

Preheat the oven to 400 degrees F.

Line the shell with parchment paper and fill with pie weights. Bake for 10 minutes. Remove the paper and bake until golden brown, about 10 to 15 minutes. Remove from the oven and cool on a wire rack.

Filling: In the top of a double-boiler, or in a metal bowl set over a pot of barely simmering water, melt the bittersweet and unsweetened chocolates and butter, stirring, until smooth. Remove from the heat.

In a bowl, beat the egg yolks with the sugar until thick and pale yellow.

In a medium, heavy saucepan, heat milk. Add 1 cup of the hot milk mixture to the yolks and whisk until smooth. Add the beaten yolk mixture to the saucepan with the hot milk and bring to a simmer over medium-low heat. Whisk in the cornstarch and salt and cook over very low heat until thickened. Simmer, whisking constantly, until thick and smooth, about 3 minutes. Remove from the heat and whisk in the melted chocolate and vanilla. Cover with plastic wrap, pressing down on the surface to prevent a skin from forming, and cool completely. Pour the filling into the crust and refrigerate until cold, at least 4 hours or overnight.

Meringue: In a large non-reactive bowl using an electric mixer, beat the egg whites with the cream of tartar and salt until soft peaks start to form. Slowly add the sugar while beating and continue to beat until glossy and stiff peaks form. (Be careful not to overbeat, as will be difficult to spread.)

Preheat the oven to 325 degrees F.

Spread the meringue over the chocolate pie, smoothing out to the pastry edges (so the meringue won't draw up during baking). With a dull knife, make decorative peaks in the meringue. Bake until the meringue is golden, about 14 to 15 minutes.

Remove from the oven and let cool for at least 2 hours in the refrigerator. Sprinkle with grated chocolate, and serve.