Preheat the oven to 350°F. Generously spray a 9-inch pie dish with cooking spray and set aside.
In the bowl of a food processor, place the Oreo cookies and pulse until finely ground. Add the melted butter and pulse until moistened. Press the mixture into the bottom and up the sides of the prepared pie dish. Bake for 11 to 12 minutes, or until fragrant and set. Let cool completely on a wire rack.
Make the filling:
In a medium saucepan, mix the granulated sugar, cocoa, cornstarch, and salt.
Gradually add about ½ cup (120 grams) of the milk, whisking the mixture until combined. Don’t worry if there are clumps. Whisk in the rest of the milk. Place over medium-high heat and cook, whisking constantly, until the mixture comes to a boil, about 7 to 9 minutes. Once boiling, continue to cook and whisk for about 45 seconds more. The mixture should be thick like pudding in order to set properly when chilled.
Immediately remove from the heat and stir in both chopped chocolates, butter, and vanilla extract until smooth. Carefully pour the hot filling into the center of the cooled crust. Avoid smoothing with a spatula which creates lines on the pie.
Cool on a wire rack until just warm, about 90 minutes.
Refrigerate, uncovered, until very cold and solid, at least 6 hours, preferably overnight.***
The untopped pie can be made up to 2 days in advance. If storing longer than overnight, cover with plastic wrap to protect from fridge odors.
Make the topping:
In a medium bowl, use an electric mixer fitted with the whisk attachment to beat the cream on medium speed until it starts to thicken, about 1 minute. Add in the sugar and vanilla and continue whipping until stiff peaks form, about 1 more minute.
Spread the whipped cream over the chilled pie. Garnish with cocoa or chocolate curls. Use a sharp knife to slice and a pie server to serve.
The finished pie can be stored in the fridge for up to 1 day. After that, the whipped topping will begin weeping and the Oreo crust will become more soggy, but will still be safe to eat.
Notes
* Both natural and Dutch-process cocoa work here. Dutch-process gives a darker color (shown in recipe photos) and smoother chocolate flavor, while natural cocoa tastes slightly sharper and more classic.** For quick chocolate curls, use a cheese slicer to gently shave the edge of a chocolate bar.*** If condensation forms on top of the pie, gently blot the surface with toilet paper from a new roll to absorb the moisture (yes, toilet paper! Paper towels don’t work as well). Do this right before adding the whipped cream topping, or up to a few hours before serving.
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