For the Crust

1. Preheat the oven to 350°F and set an oven rack in the middle position.
2. In the bowl of a food processor fitted with a steel blade, blend the Oreos and butter until the mixture is finely chopped and evenly moistened. Firmly press the crumbs into a deep-dish 9-inch pie pan, starting by lining the sides to create a crust about ¼-inch thick, then press the remaining mixture onto the bottom to form the base. Use the bottom of a measuring cup or glass not only to flatten and even out the surface of the base but also to press against the sides, ensuring they are compact and uniform. To refine the top edge of your crust, position the side of your thumb or the edge of a measuring cup inside the crust's side wall, and with the index finger of your other hand, gently press down on the top edge. Gradually rotate the pie pan, maintaining this motion, to ensure a consistently neat and smooth edge around the entire perimeter. Bake the crust for 5 to 6 minutes, or until slightly crisp, then allow it to cool completely before filling.

For the Filling

1. Combine the sugar, cornstarch, and salt in a heavy medium saucepan. Whisk gently until combined.
2. In a bowl or large measuring cup, whisk together the egg yolks and milk. Add the milk mixture to the saucepan in a slow and steady stream, whisking until the mixture is smooth. Cook over medium-high heat, whisking frequently, until the mixture starts to bubble and thicken, 6 to 8 minutes. Immediately turn the heat down to a simmer and cook -- whisking constantly, especially around the edges -- for one minute more, until thick. (The constant stirring and low heat prevent the eggs from scrambling. Be sure to scrape the whisk against the bottom and edges of the pan where the mixture is more likely to overheat.) Off the heat, immediately add the chopped chocolate, butter, and vanilla extract; whisk until the chocolate is completely melted and the mixture is evenly combined.
3. Pour the filling into the crust and smooth the top. Press a piece of saran wrap directly over the surface of the filling to prevent a film from forming; chill in the refrigerator for at least 8 hours or overnight.

For the Topping

1. Up to 3 hours before serving, place the heavy cream in a mixing bowl. Using an electric mixer, whip on medium-low speed until the whisk or beaters begin to leave tracks in the cream. Add the sugar and whip until the cream holds medium, silky peaks. Go slowly towards the end; if it gets grainy or curdled, you’ve gone too far. Spread the whipped cream over the filling, leaving some of the chocolate filling showing around the edges. Sprinkle with the grated chocolate and refrigerate until ready to serve.
2. Make-Ahead Instructions: The pie can be assembled, without the whipped topping, up to 1 day ahead of time. Store in the refrigerator covered with plastic wrap.