Blend Oreos and salt in a food processor until you have a fine crumb. Add melted butter and blend again for 5 seconds
Press cookie crumbs into the bottom and sides of a 9 inch pie pan then put crust in freezer until ready to fill
Whisk egg whites and sugar in a medium heatproof bowl over a double boiler until sugar dissolves. Do not allow egg whites to become hot. This takes about 3 minutes
Remove bowl from the double boiler and use a hand mixer to beat egg whites for 7-8 minutes on high speed. Egg whites will become fluffy, and form stiff peaks
In a separate medium sized heat proof bowl, melt chocolate and butter over the same double boiler, stirring frequently with a rubber spatula until smooth
In another bowl, beat 1½ cups COLD heavy cream with salt and vanilla on medium-high speed until medium peaks form
Fold egg white mixture into chocolate mixture, working quickly to avoid chocolate becoming gritty or chunky
Fold whipped cream into chocolate mixture just until no white streaks are visible
Add chocolate mousse mixture into chilled pie crust
Beat 1½ cups heavy cream, sour cream and kosher salt in a medium bowl until medium peaks form. Spread mixture over the top of the chocolate mousse mixture then chill AT LEAST 3 hours, or overnight
Top pie with chocolate shavings or a very light dusting of cocoa powder
