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 1 at a time, add the yolks, 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 Grand Marnier and continue beating until it holds soft peaks.
Fold the egg whites into the chocolate mixture until no white speaks appear. Gradually fold in the whipped cream, reserving about 1/2 cup for garnish.
Transfer to a large decorative silver or glass bowl and refrigerate until well chilled.
To serve, spoon the reserved whipped cream on top and garnish with chocolate shavings.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.