For pastry dough:
- 2 1/2 cups flour
- 1 tablespoon sugar
- 1 teaspoon salt
- 2 sticks (8 ounces, 16 tablespoons or 1 cup) unsalted butter, very cold
- 1/4 cup cornstarch
- 1/3 cup plus 2 tablespoons sugar, divided
- 3 tablespoons unsweetened cocoa powder
- 1/4 teaspoon salt
- 3 cups whole milk
- 4 oz bittersweet chocolate (not more than 60% cacao), finely chopped
- 1 teaspoon pure vanilla extract
- 1 cup chilled heavy cream
- bittersweet chocolate shavings (optional)
Gather your ingredients: Fill a one cup liquid measuring cup with water, and drop in a few ice cubes; set it aside. In a large bowl — I like to use a very wide one, so I can get my hands in — whisk together 2 1/2 cups flour, 1 tablespoon of sugar and a teaspoon of salt. Dice two sticks (8 ounces or 1 cup) of very cold unsalted butter into 1/2-inch pieces. Get out your pastry blender.
Sprinkle the butter cubes over the flour and begin working them in with the pastry blender, using it to scoop and redistribute the mixture as needed so all parts are worked evenly. When all of the butter pieces are the size of tiny peas — this won’t take long — stop. Yes, even if it looks uneven; you’ll thank me later.
Glue it together: Start by drizzling 1/2 cup of the ice-cold water (but not the cubes, if there are any left!) over the butter and flour mixture. Using a rubber or silicon spatula, gather the dough together. You’ll probably need an additional 1/4 cup of cold water to bring it together, but add it a tablespoon as a time. Once you’re pulling large clumps with the spatula, take it out and get your hands in there (see how that big bowl comes in handy?). Gather the disparate damp clumps together into one mound, kneading them gently together.
Pack it up: Divide the dough in half, and place each half on a large piece of plastic wrap. I like to use the sides to pull in the dough and shape it into a disk. Let the dough chill in the fridge for one hour, but preferably at least two, before rolling it out.
Do ahead: Dough will keep in the fridge for about a week, and in the freezer longer. If not using it that day, wrap it in additional layers of plastic wrap to protect it from fridge/freezer smells. To defrost your dough, move it to the fridge for one day before using it.
Make pie shell:
Roll out dough on a lightly floured surface with a lightly floured rolling pin into an 11-inch round, then fit into a 9-inch pie plate. Trim edge, leaving a 1/2-inch overhang, then fold overhang under and crimp edge decoratively. Prick bottom and side of shell all over with a fork, then chill shell 30 minutes.
While shell chills, preheat oven to 375°F with a baking sheet on middle rack.
Line shell with foil and fill with pie weights. Bake on baking sheet until pastry is set and edge is pale golden, about 25 minutes. Carefully remove weights and foil, then bake shell on baking sheet until pale golden all over, 15 to 20 minutes more. Cool shell.
Whisk together cornstarch, 1/3 cup sugar, cocoa powder, and salt in a 2-qt heavy saucepan, then gradually whisk in milk. Bring to a boil over medium heat, whisking constantly, then boil, whisking, 2 minutes (mixture will thicken). Remove from heat and whisk in chocolate and vanilla until smooth.
Pour filling into cooled shell and chill, its surface covered with wax paper (if you want to prevent a skin from forming), until cold, at least 2 hours.
Just before serving, beat cream with remaining 2 Tbsp sugar until it just holds soft peaks. Spoon onto pie.
Pie (without whipped cream) can be chilled up to 1 day.