Preheat oven to 350°F (177°C).

In a food processor or blender, pulse 22 Oreos (including the cream filling) into a fine crumb. You should have about 2 cups (packed) crumbs, or 250g. Pour crumbs into a large bowl. Add the melted butter and stir to combine. The mixture will be thick and quite wet. Try to smash/break up any large chunks. Pour the mixture into an ungreased 9-inch pie dish. With medium pressure using your hand, pat the crumbs down into the bottom and up the sides to make a compact, thick crust. Bake for 10 minutes. Allow to cool as you prepare the filling.

In a medium heatproof bowl with a pour spout, whisk the egg yolks and cornstarch together with a fork or small whisk. It will be very thick. Set aside.

Combine the milk, heavy cream, sugar, and salt in a medium saucepan set over medium heat. Whisk until all the sugar has dissolved, then bring to a gentle simmer, whisking occasionally.

Once simmering, remove about ½ cup (120ml) of the warm milk mixture and, in a slow and steady stream, whisk into the egg yolk and cornstarch mixture. Keep those egg yolks moving so they don’t scramble. In a slow and steady stream, pour and whisk the egg yolk mixture into the saucepan. The pudding will immediately begin to bubble and thicken. Cook for 1 full minute, whisking constantly.

Remove the pan from heat and whisk in the butter. Then whisk in the chopped chocolate, espresso powder, and vanilla. Allow to cool for 10 minutes.

Spread filling into cooled pie crust. Cover tightly with plastic wrap and refrigerate for at least 4–6 hours or overnight, until chilled and thickened.

In a very small bowl, using a fork, mix the espresso powder and warm water together. Allow to cool down for a few minutes. Using a hand mixer or a stand mixer fitted with a whisk attachment, whip the cold heavy cream, sugar, cocoa powder, and espresso mixture on medium-high speed until medium peaks form, about 3 minutes. Spread onto chilled pie. Garnish with chocolate curls, chips, or chocolate-covered coffee beans, if desired.

Slice and serve. Cover leftover pie and store in the refrigerator for up to 5 days.