Crush the chocolate sandwich cookies into fine crumbs in a food processor, mix with the melted butter, and press firmly into the bottom and sides of a 9-inch pie dish. Freeze for 15 minutes.
Melt the semisweet chocolate in the microwave in 30-second bursts, stirring between each, then let it cool for 10 minutes until room temperature but still pourable.
Beat the softened butter, powdered sugar, and salt on medium-high speed for 3 minutes, until pale and fluffy.
Beat in the cooled melted chocolate and vanilla until the mixture is completely silky.
In a separate bowl, whip 1 1/2 cups of cold heavy cream to stiff peaks, then fold it into the chocolate mixture in three additions until no streaks remain.
Spread the filling into the chilled crust, smooth the top, and refrigerate for at least 4 hours or overnight until set.
Whip the remaining 1 cup of cream with 2 tbsp powdered sugar to medium peaks and spread or pipe it over the pie.
Shave the dark chocolate over the top with a vegetable peeler and serve chilled - the filling is unbelievably smooth and silky.
