Crush the Oreo cookies into fine crumbs using a food processor or resealable bag and rolling pin
Mix the crumbs with melted butter until evenly combined.
Press the mixture firmly into a 9-inch pie dish, covering the bottom and slightly up the sides. Refrigerate while preparing the filling.
Place the chopped chocolate, butter, and water in a microwave-safe bowl. Heat in short intervals, stirring between each one, until smooth.
Stir in the vanilla and let the chocolate cool until slightly warm.
Beat the cold heavy cream with powdered sugar until soft to medium peaks form
Gently fold about one-third of the whipped cream into the cooled chocolate mixture
Fold in the remaining whipped cream in two additions, using gentle movements to preserve the mousse's airy texture
Spoon the chocolate mousse into the chilled Oreo crust and smooth the top
Cover and refrigerate for at least 4 hours, preferably overnight.
Before serving, whip the remaining heavy cream with powdered sugar and vanilla until firm peaks form
Spread or pipe the whipped cream over the chilled pie
Finish with crushed Oreos and chocolate shavings, if desired
Slice with a sharp knife and serve cold.
