For the chocolate cookie crust:
- Heat the oven to 350°F and arrange a rack in the middle.
- Place the cookies in a food processor fitted with a blade attachment and process until the pieces are about the size of peas. Stop the motor, add the melted butter, and continue to process until the crumbs are fine, about the size of coarsely ground coffee (you will need 1 1/2 cups). (Alternatively, place the cookies in a resealable plastic bag, press out the air, and seal. Using a rolling pin, smash into uniform fine crumbs. Transfer to a medium bowl, add the melted butter, and mix until evenly combined.)
- Pour the crumb mixture into a 9-inch pie plate and, using the bottom of a cup or your fingers, press firmly and evenly into the bottom and up the sides. Bake until fragrant, about 10 to 15 minutes. Remove to a wire rack to cool completely before making the filling, about 40 minutes.
For the chocolate mousse:
- Fill a medium saucepan with 2 inches of water and bring to a simmer over medium heat.
- Place the chopped chocolate and 1/4 cup of the cream in a large heatproof bowl. (Place the remaining cream back in the refrigerator until ready to use.) Nest the bowl over the saucepan, making sure the bottom of the bowl isn’t touching the water. Melt the chocolate, stirring occasionally with a rubber spatula, until smooth and combined with the cream. Remove the bowl from the saucepan, wipe any moisture from the bottom of it, and set aside to cool slightly.
- Place the egg whites in the bowl of a stand mixer fitted with a whisk attachment (make sure the bowl and whisk have no trace of oil or fat on them, or the whites won’t whip properly). Mix on high until stiff peaks form, about 1 minute; transfer to a medium bowl and set aside. (Alternatively, you can whip the whites by hand in a large bowl for about 3 minutes.)
- Clean and dry the whisk attachment and mixer bowl. Place the remaining 3/4 cup of cream in the bowl and whisk on high speed until stiff peaks form, about 1 minute. (Alternatively, you can whisk the cream by hand in a large bowl for about 3 minutes.)
- Using a rubber spatula, fold half of the whipped cream into the melted chocolate, then gently stir in the rest (try not to deflate the whipped cream). Gently fold the egg whites into the chocolate-cream mixture just until there are no longer large blobs of whipped cream or egg white (do not overmix). Spoon the mousse into the cooled pie crust and smooth it into an even layer. Refrigerate uncovered until set, at least 2 hours.
For the whipped cream:
- When ready to serve, place the bowl of the stand mixer and the whisk attachment in the freezer for 10 minutes. Add the sugar, vanilla, and cream to the bowl and whisk on high speed until medium peaks form, about 1 to 2 minutes. (Alternatively, you can whisk the cream by hand in a large chilled bowl for about 3 to 4 minutes.) Spread or decoratively pipe the whipped cream from a pastry bag evenly over the pie.
- To garnish, hold the piece of chocolate with a paper towel in one hand and use a vegetable peeler to shave the desired amount over the top of the pie.