Prepare the Crust:

1. Make the dough: In a large bowl, whisk together flour, salt, and sugar. Add cold, cubed butter and use a pastry cutter or your fingers to work the butter into the flour until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining.
2. Add water: Sprinkle 3 tablespoons of ice water over the mixture and stir with a fork until the dough starts to come together. Add the remaining tablespoon of water if needed. The dough should hold together when pressed but not feel wet.
3. Chill: Form the dough into a disk, wrap in plastic wrap, and refrigerate for at least 30 minutes (or use store-bought crust to skip steps 1-3).
4. Roll and shape: On a lightly floured surface, roll the chilled dough into a 12-inch circle. Transfer to a 9-inch pie dish, trim excess dough leaving a 1-inch overhang, and fold the edges under. Crimp decoratively. Prick the bottom with a fork.
5. Blind bake: Preheat oven to 375°F (190°C). Line the crust with parchment paper and fill with pie weights or dried beans. Bake for 15 minutes, remove weights and parchment, and bake another 8-10 minutes until golden brown. Cool completely.

Make the Chocolate Filling:

6. Combine dry ingredients: In a medium bowl, whisk together sugar, cornstarch, cocoa powder, and salt until no lumps remain.
7. Mix egg yolks: In a separate bowl, whisk the egg yolks until smooth. Set aside.
8. Heat milk and cream: In a medium saucepan over medium heat, combine milk and heavy cream. Heat until steaming and small bubbles form around the edges (do not boil).
9. Temper eggs: Gradually whisk about 1 cup of the hot milk mixture into the egg yolks to warm them gently. Then whisk the egg mixture back into the saucepan with the remaining milk.
10. Cook custard: Add the sugar-cornstarch mixture to the saucepan and whisk constantly over medium heat. Continue whisking as the mixture thickens, about 5-7 minutes. It should coat the back of a spoon and maintain slow bubbles.
11. Add chocolate: Remove from heat and immediately add the chopped chocolate, butter, and vanilla extract. Whisk until the chocolate melts completely and the mixture is silky smooth.
12. Strain and cool: Pour the filling through a fine-mesh strainer into a bowl to remove any lumps, then pour into the cooled pie crust. Press plastic wrap directly onto the surface of the filling to prevent a skin from forming.
13. Chill: Refrigerate the pie for at least 4 hours or overnight until the filling is completely set and cold.

Prepare the Topping:

14. Whip cream: When ready to serve, add cold heavy cream, powdered sugar, and vanilla extract to a large mixing bowl. Beat with an electric mixer on medium-high speed until stiff peaks form, about 3-4 minutes.
15. Top and garnish: Spread or pipe the whipped cream over the chilled chocolate filling. Garnish with chocolate shavings or a light dusting of cocoa powder if desired.
16. Serve: Slice with a sharp knife, wiping it clean between cuts for neat slices. Serve immediately and refrigerate any leftovers.