Preheat the oven to 350°F (177°C).

Crush Oreo cookies (with filling) in a food processor until finely ground.

Combine Oreo crumbs and melted butter in a bowl, mixing until well combined.

Press the crumb mixture into a 9-inch pie dish, forming a crust.

Bake for 10 minutes. Cool completely.

Whisk egg yolks and cornstarch in a heatproof bowl until thick.

Combine salt, sugar, heavy cream, and milk in a saucepan.

Heat over medium, whisking until sugar dissolves and the mixture simmers.

Slowly whisk half a cup of the hot milk mixture into the egg yolks.

Pour the egg yolk mixture back into the saucepan, whisking constantly.

Whisk for 1 minute after the mixture bubbles and thickens.

Remove from heat and whisk in butter, chopped chocolate, espresso powder, and vanilla.

Cool filling for 8–10 minutes.

Pour the filling into the cooled crust.

Cover with plastic wrap and chill for at least 5–6 hours or overnight.

Dissolve espresso powder in a small amount of warm water and cool.

Whip together the cooled espresso mixture, cocoa powder, heavy cream, and sugar until medium peaks form.

Spread whipped cream over chilled pie and garnish as desired.

Slice and serve.