Preheat oven to 350°F (175°C). In a medium bowl combine 1 1/2 cups graham cracker crumbs, 1/4 cup unsweetened cocoa powder, 1/4 cup granulated sugar, and 1/4 tsp fine salt. Stir to combine.

Add butter and press

Melt 6 tbs unsalted butter and pour over dry mixture. Stir until evenly moistened. Press the crumb mixture firmly and evenly into the bottom and up the sides of a 9-inch (23 cm) pie dish to form a 3/8"–1/2" thick crust.

Blind bake

Bake crust in preheated oven for 10 minutes until set. Remove from oven and let cool completely on a wire rack. Reduce oven temperature to 300°F (150°C) if you prefer to reheat later for a short bake of the filled pie (not required).

Chocolate Cream Filling

Warm dairy and dry ingredients

In a medium saucepan whisk together 1 cup whole milk, 1 cup heavy cream, 1/2 cup granulated sugar, 1/3 cup unsweetened cocoa powder, and 3 tsp cornstarch until smooth and free of lumps.

Temper egg yolks

Place 3 egg yolks in a small bowl and whisk lightly. Heat the milk-cream mixture over medium heat, stirring constantly, until it is hot and just beginning to steam but not boiling (small bubbles at edges). Remove about 1/3 cup of the hot mixture and whisk it into the yolks to temper them.

Cook the custard

Pour the tempered yolk mixture back into the saucepan and return to medium-low heat. Cook, stirring constantly with a heatproof spatula, until mixture thickens and coats the back of the spatula (170–175°F / 77–80°C), about 3–5 minutes. Do not boil.

Melt chocolate

Remove pan from heat. Add 4 oz chopped semisweet chocolate, 2 tbs unsalted butter, 1 tsp vanilla extract, and 1/8 tsp fine salt. Stir until chocolate and butter are fully melted and the filling is glossy and homogeneous.

Strain and cool

For the silkiest texture, strain the filling through a fine-mesh sieve into a clean bowl to remove any cooked egg bits or cocoa lumps. Cover the surface with plastic wrap to prevent a skin from forming and let cool to warm (about 20–30 minutes).

Fill the crust

When the crust is completely cooled and the filling is slightly warm (not hot), pour the chocolate cream into the prepared crust and smooth the top with an offset spatula. Chill in the refrigerator at least 4 hours, preferably overnight, until firmly set.

Whipped Topping

Whip cream

Just before serving, chill a mixing bowl and beaters for 10 minutes for best results. Pour 1 cup cold heavy cream into the bowl, add 2 tbs powdered sugar and 1/2 tsp vanilla extract. Whip on medium-high speed until soft peaks form, about 2–3 minutes. Do not overwhip.

Top the pie

Spread or pipe the whipped cream over the chilled chocolate filling in an even layer or decorative swirls. Return briefly to the fridge if needed to firm the topping (10–15 minutes).

Garnish (optional)

Prepare garnishes

If using toasted cornflakes, spread 1/2 cup cornflakes on a baking sheet and toast in a 325°F (160°C) oven for 4–6 minutes until lightly golden, watching closely so they don't burn. Let cool.

Finish

Sprinkle chocolate shavings (about 2 tbs) and toasted cornflakes across the whipped topping just before serving to evoke the "cornet" idea (crunch + chocolate). Slice and serve chilled.