Prepare the crust: Mix chocolate wafer crumbs, 1/4 cup sugar, and melted butter until evenly moistened. Press the mixture firmly into the bottom and up the sides of a 9-inch pie plate. Chill in the refrigerator for 30 minutes.
Melt the chocolate: Place chopped bittersweet chocolate in a heatproof bowl. Pour 1/4 cup hot water over the chocolate and let it sit for 2 minutes, then stir until smooth. Stir in 1 teaspoon vanilla extract. Let cool slightly.
Whip the mousse base: In a separate bowl, whip 1 cup of cold heavy cream until soft peaks form. Set aside.
Make the meringue: In a clean, grease-free bowl, beat the egg whites until foamy. Gradually add 1/4 cup sugar and continue beating until stiff, glossy peaks form.
Combine the mousse: Gently fold the slightly cooled melted chocolate mixture into the whipped cream until just combined. Then, gently fold the meringue into the chocolate-cream mixture until no white streaks remain, keeping the mixture light.
Fill the pie: Pour the mousse filling into the chilled crust. Smooth the top with a spatula. Refrigerate for at least 6 hours, or preferably overnight, until the filling is firm.
Prepare the topping: Before serving, whip the remaining 1 cup of cold heavy cream with 2 tablespoons powdered sugar and 1/2 teaspoon vanilla extract until stiff peaks form.
Finish the pie: Spread or pipe the whipped cream over the chilled pie. Garnish with chocolate shavings if desired. Slice and serve cold.
Notes
For a firmer crust, you can bake the crust at 350 degrees F for 8 minutes before chilling.
To stabilize the whipped cream topping, add 1 teaspoon of unflavored gelatin dissolved in 2 tablespoons of cold water to the cream before whipping.
Use high-quality dark chocolate for the best flavor in the mousse.