In the top of a double boiler or in a bowl set over a pot of hot water, melt the chocolate and butter, stirring. Remove from the heat and beat with a heavy wooden spoon until smooth. Return to the heat and add the yolks, 1 at a time, beating well after the addition of each. Remove from the heat and transfer to a large bowl.
In a clean bowl, beat the egg whites until soft peaks start to form. Add 2 tablespoons of the sugar and beat until stiff.
In a third bowl, beat the cream until it becomes frothy. Add the remaining 2 tablespoons sugar and the orange liqueur and continue beating until it holds soft peaks.
Fold the egg whites into the chocolate mixture until no white specks appear. Gradually fold in the whipped cream, reserving about 1/2 cup for garnish.
Spoon the mousse into the pre-baked pie shell and smooth the top with a rubber spatula. Refrigerate until well chilled.
To serve, spoon the reserved whipped cream on top and garnish with chocolate shavings. Cut into wedges and serve.
Sift the flour, sugar, and salt into a large mixing bowl. Using a pastry blender or your fingers, work in the butter pieces and shortening until the dough begins to come together and form small pea shapes. Work in the ice water with your fingers until it just comes together, being careful not to over mix. Form the crust into a disk shape, wrap tightly in plastic wrap, and place in the refrigerator to rest for at least 30 minutes before rolling out to fit into a pie pan.
Preheat the oven to 400 degrees F.
On a lightly floured surface, roll out the dough to an 11-inch circle. Transfer to a 10-inch tart pan with a removable bottom and trim any excess from the edges. Place in the refrigerator for 20 to 30 minutes to rest. Cover with parchment paper and weight with pie weights. Bake for 12 to 15 minutes. Remove the paper and weights and cook until just golden brown, about 15 minutes. Remove from the oven and cool.