Pulse the cookies in a food processor until finely chopped. With the processor running, add the butter. Press the crumbs into the bottoms of four 4 1/4-inch disposable aluminum pie pans. Bake until set, about 5 minutes. Let cool.
For the filling: Whisk together the sugar, cornstarch and salt in a medium saucepan. Whisk in the milk and egg yolks. Stir over medium heat until the mixture just barely comes to a boil and becomes thick, 6 to 8 minutes. The second it starts to bubble and thicken (it should be thick like pudding), remove it from the heat. Add the chocolate, butter and vanilla, then stir until everything is beautifully combined. Pour the pudding into the pie crusts and place in the fridge, uncovered, to chill for 4 hours. Serve topped with whipped cream.