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.