Melt chocolate and butter carefully in the microwave, or use a double boiler. Add boiling water and set aside to cool slightly.
Whip cold heavy cream until soft peaks form. Place in refrigerator.
Whip egg whites and sugar in a separate bowl until soft peaks form.
When the melted chocolate has cooled to approximately 100 degrees F, stir in the egg yolks.
Fold half of the whipped cream into the chocolate mixture. Fold half of the egg whites into the chocolate mixture.
Fold in the other half of the whipped cream and the other half of the egg whites.
For the crust, either use a pre-made Oreo Cookie Crust OR crush 25 Oreo cookies and mix with 5 tablespoons melted butter then press into bottom and sides of pie pan.
For the whipped cream, mix heavy whipping cream, powdered sugar, and vanilla until medium to stiff peaks form.
To put it all together, spoon chocolate mousse into pie crust. Smooth top. Refrigerate for at least 1 hour before topping with whipped cream.