Melt the chocolate and 1 cup of cream in a double boiler until chocolate has melted and the cream is incorporated. Meanwhile, whip the egg whites and sugar until soft peaks form. Remove egg whites to a large mixing bowl. Whip the remaining 3 cups of cream to soft peaks. Let the melted chocolate cool slightly.
Fold the chocolate into the whipped cream. Fold the beaten egg whites into the chocolate and whipped cream mixture.
Refrigerate the mousse for a few hours or until it has been set up. Proceed to layer the chocolate cake.
Layer the chocolate mousse on a piece of cake. Place one piece of cake on top of that. Layer with mousse and place another piece of cake on top to make 3 layers of cake with 2 layers of mousse. Repeat with the remaining pieces of cake and mousse.
Wrap the layered cake in plastic and freeze. When ready to serve, trim off the edges of the cake. Cut the cake into triangles 2 inches wide and 4 inches tall.