heat oven to 350F and turn oreos to crumbs, if needed
mix butter into crumbs until evenly moistened (food processor helps, especially if making the crumbs yourself)
press crumbs into bottom and sides of 9“ pie dish, bake until set, about 12min, cool
prepare a fine mesh sieve over a medium bowl
in a medium saucepan, whisk sugar, cornstarch, cocoa powder, and salt
pour in a little milk and which to smooth paste, add remaining milk gradually to mix evenly
whisk in egg yolks
cook over medium heat, whisking constantly, until it thickens and comes to a low boil, about 8-9min
cook and stir 1min more then pour through sieve, discard any solids
stir in chocolate, butter, and vanilla, let stand 1min to melt, then stir until smooth
pour custard into crust, let cool slightly, cover surface with plastic wrap and chill until firm, at least 6hr
to serve, whip cream and powdered sugar to soft peaks and top pie