Place the graham crackers, sugar, cornstarch, cinnamon and salt in a small food processor. Pulse until the ingredients are evenly combined and a fine crumb.
150 g graham crackers, 67 g sugar, 3 g cornstarch, 1 g 1/4 tsp cinnamon, Pinch of salt
Add the butter, pulse until the mixture resembles wet, coarse sand.
70 g butter
Press the crust mixture into the bottom and up the sides of a 9 1/2” or 10" deep dish pie plate
Bake at 350° F for 10 minutes, remove from the oven and brush lightly with the beaten egg white, return to the oven and bake for an additional 2 minutes to set the egg white (the egg white helps seal the crust and keep it from becoming soggy from contact with the custard filling, if you choose not to use the egg white, simply bake the crust for 12 minutes)
1 egg white
Make the filling:
Place 960ml (or 4 cups) of the milk in a medium saucepan. Add 88g (or 7 tbsp) of sugar and bring the mixture to a boil.
1080 ml milk, 176 g sugar
While the milk is heating, whisk the remaining 88g (or 7 tbsp) of sugar, cornstarch, cocoa and salt together in a medium bowl. Add the eggs and egg yolks and whisk to thoroughly combined. Add the remaining 120ml (1/2 cup) milk and whisk until fully incorporated.
176 g sugar, 40 g cornstarch, 24 g cocoa powder, 3/4 tsp salt, 2 large eggs, 4 large egg yolks, 1080 ml milk
Temper the egg mixture by slowly and continuously whisking about 1 cup of the heated milk mixture into it.
Slowly add the tempered egg mixture into the heated milk mixture in the saucepan, whisking continuously.
Continue whisking and cooking the mixture until it thickens and you can see defined trails of the whisk through the mixture (5-10 minutes). The mixture should bubble a bit, but should not reach a full boil.
Remove from the heat, add the melted chocolate, butter and vanilla. Blend with an immersion blender until the butter is fully melted and incorporated and the mixture is silky smooth.
340 g bittersweet chocolate, 56 g unsalted butter, 15 ml vanilla extract
Allow to cool slightly (about 5 minutes) stirring occasionally to prevent a skin from forming on the surface.
Pour the custard into the crust - if you prefer to avoid a skin forming on the top, you can place a layer of plastic wrap directly on the surface.
Refrigerate until set (at least 4 hours).
Make the topping:
Place 180ml (3/4 cup) whipping cream, the white chocolate and the salt in a microwave safe bowl or a double boiler. Heat, stirring frequently, until the white chocolate is completely melted forming a loose white chocolate ganache. If you're doing this in the microwave, use a built in chocolate melting setting, or heat in 30 second bursts stirring in between until the chocolate is completely melted.
360 ml whipping cream, 170 g white chocolate, pinch salt
Cover the white chocolate ganache and chill overnight - the mixture will be soft and stir-able straight from the refrigerator.
Using an electric mixer, beat the white chocolate ganache until it is light and fluffy - avoid over beating or the mixture may curdle or break.
Pour the remaining 180ml (3/4 cup) of whipping cream into the whipped ganache and resume whipping (start low and work up to medium speed) until soft to medium peaks form.
360 ml whipping cream
You can cover the entire top of the pie with the topping, or serve individual slices with a dollop of the topping. If you'd like, you can garnish with chocolate curls, shaving, or sprinkles.
Notes
This pie can be a bit time consuming to make, although most of that is down time waiting for elements to chill. You can mitigate that by making some elements, like the crust and the white chocolate ganache, several days in advance if you like.
I used a deep dish 10 inch pie plate for this recipe. However, "deep dish" is not a standardized size so the depth of these pie plates often varies from brand to brand. If your plate isn't quite as deep as mine, or is 9 1/2 inch rather than 10 inch, you may have a little more filling than will fit. Simply pour this into a small ramekin and enjoy it on its own.