Add cold, cubed unsalted butter to the dry mix. Using a pastry cutter or your fingertips, work the butter into the flour until the mixture resembles coarse crumbs with pea-sized pieces of butter remaining.

Form dough

Sprinkle cold water and vanilla extract over the crumb mixture a tablespoon at a time. Use a fork to bring the dough together just until it holds when squeezed. Do not overwork. Flatten into a disk, wrap in plastic, and chill in the refrigerator for 30 minutes.

Roll and shape

Preheat oven to 350°F (175°C). On a lightly floured surface, roll the chilled dough to about 1/8-inch thickness. Cut a circle large enough to fit a 9-inch tart/ pie pan with 1/2-inch overhang, or, if creating individual cornet shapes, cut into 6–8 circles or ovals. Gently press dough into the pan to form an even crust. Trim excess, fold/ crimp edges.

Blind bake crust

Line the crust with parchment and fill with pie weights or dried beans. Bake 15 minutes, remove weights and parchment, then bake an additional 8–10 minutes until set and slightly crisp. Let cool completely on a rack before filling.

Chocolate Cream Filling

Warm dairy

In a medium saucepan, combine whole milk and heavy cream. Warm over medium heat until just simmering (do not boil), stirring occasionally.

Mix dry custard base

While the milk warms, whisk together granulated sugar, cornstarch, cocoa powder, and salt in a bowl. In a separate small bowl, lightly beat the egg yolks.

Temper eggs

Slowly ladle about 1/2 cup of the hot milk mixture into the beaten yolks while whisking constantly to temper. Then pour the tempered yolks back into the saucepan with the remaining milk, whisking to combine.

Thicken custard

Return the pan to medium heat and cook, whisking constantly, until the mixture thickens and comes to a gentle boil (about 2–4 minutes). Continue to cook for 1 minute at a low boil to fully activate the cornstarch. Remove from heat.

Finish filling

Stir in the butter, chopped semi-sweet chocolate, and vanilla. Whisk until smooth and fully incorporated. For an extra-silky texture, pass the hot custard through a fine-mesh sieve into a clean bowl to remove any lumps.

Cool

Place a piece of plastic wrap directly on the surface of the custard to prevent a skin from forming. Chill in the refrigerator until cold, at least 2 hours (or up to overnight) so it firms enough to pipe if desired.

Whipped Cream Topping

Whip

Just before serving, pour cold heavy whipping cream into a chilled bowl. Add powdered sugar and vanilla. Whip with a hand or stand mixer on medium-high speed until soft to medium peaks form. Do not overbeat.

Top

Fold or pipe the whipped cream over the chilled chocolate filling using a spatula or piping bag fitted with a star tip for a decorative cornet-style top.

Garnish & Serve

Garnish

Sprinkle grated chocolate or chocolate shavings over the whipped cream and lightly dust with cocoa powder.

Serve

Slice the pie and serve chilled. Store any leftovers covered in the refrigerator for up to 3 days.