Mile-High Chocolate Pie

Preheat oven to 350°F. Line pie dish with crust; crimp edges decoratively. Fully bake pie crust according to recipe or box instructions. Let cool completely.
Whisk egg whites and sugar in a medium heatproof bowl set over a medium saucepan of simmering water until sugar dissolves and egg whites are warm but not hot, 3-4 minutes. Remove from heat. Using an electric mixer, beat on medium-high speed until cool, tripled in volume, and stiff peaks form (the tips of the peaks won't fall over when beaters are lifted from bowl and turned upright), about 6 minutes.
Stir chopped chocolate and butter in a large bowl set over same saucepan of simmering water until melted and smooth, 4-5 minutes; set aside.
Beat 2 cups cream in another medium bowl until medium peaks form (cream should be soft and pillowy), 5-6 minutes.
Gently fold egg whites into warm chocolate mixture until fully incorporated (work quickly to prevent chocolate from turning gritty). Gently fold in whipped cream just until no white streaks remain; do not overmix or mixture will deflate. Spoon 1/2 cup chocolate mousse into bottom of prepared pie crust; spread evenly over bottom of crust. Sprinkle 3/4 cup plus 2 tablespoons crushed chocolate wafers over mousse. Top with remaining mousse, mounding in the center to create a dome. (The point is to add height, not to spread out evenly to edges.) Chill pie.
Beat remaining 2 cups cream, crème fraîche, and salt until medium-stiff peaks form (when the beaters are lifted from the cream, the peaks will hold their shape but the tips will fall over). Top chocolate mousse with whipped-cream mixture, following the same rounded dome shape. Chill pie for at least 4 hours or, covered, for up to 3 days. (It will slice best if chilled overnight, allowing mousse to set properly.)
Garnish pie with 2 tablespoons chocolate wafers and chocolate shavings. Slice pie using a clean, dry knife; wipe between slices to ensure clean, elegant pieces.