Remove 4 Oreo cookies from the package and set aside. In a food processor (or a large ziploc bag with a rolling pin) crush remaining Oreo (filling intact) cookies until fine crumbs form. Add butter and salt, pulse or mix to combine.
Press moistened crumbs into the bottom of a 9-inch pie dish. Freeze for 30 minutes while preparing the filling
In a large bowl combine the chocolate, butter and vanilla. Place a mesh sieve over top and set aside.
To make the filling, in a medium heavy bottomed saucepan set over medium-low heat, add the milk, 1 1/2 cups heavy cream, sugar, salt, and espresso powder. Heat until the sugar has dissolved and the mixture is warm, and gently bubbling.
Meanwhile, in a medium heatproof bowl, whisk together the remaining 1/2 cup of heavy cream, cornstarch and the egg yolks until a thick paste forms.
Carefully temper the egg yolks by slowly drizzling the warm mixture into the medium bowl, whisking continuously to prevent the eggs from scrambling. It will take about 5 minutes to incorporate the mixture.
Return the mixture to the stovetop and heat until it reaches 165 degrees on an instant read thermometer AND is thick like pudding (the thicker, the better), about 5-8 minutes. Pour over the sieve to remove any overcooked pieces of egg that make have formed. Remove the sieve and whisk the milk into the chocolate until melted. As the pudding cools the mixture will thicken.
Pour warm mixture into the chilled crust. Cover with plastic wrap, directly on the pudding. Chill in the refrigerator for 4 hours or overnight until fully set. It should be firm to the touch.
When ready to serve in the bowl of a stand mixer fitted with the whisk attachment or a large mixing bowl with a hand mixer, beat the heavy cream, powdered sugar and vanilla extract until stiff peaks form. Spread over the pudding, then remove the filling from the remaining 4 Oreo cookies and crumble over top.
Keep refrigerated until ready to serve.