Pour the liqueur into the measuring cup. Grate the colored part of the skins of 3 oranges and the ½ lemon into the cup. Strain in enough orange juice so liquid measures 2 cups.

Beat the egg yolks and sugar in a mixing bowl until mixture is pale yellow and forms the ribbon. Beat in the cornstarch and the orange juice mixture. Pour into the saucepan and stir over moderate heat with wooden spoon until mixture heats through and thickens, but does not come to the simmer, or a temperature of more than 170 degrees. It should coat the spoon lightly. Remove from heat and beat a moment to stop the cooking.

Beat the egg whites and salt in a separate bowl until soft peaks are formed; sprinkle in the sugar and beat until stiff peaks are formed. Fold the egg whites into the hot orange mixture, and fold over the bowl of ice until thoroughly chilled so the custard will not separate.

Beat the cream until stiff, and fold into the chilled mousse. Turn into orange-shell cups, dessert cups, or bowl. Cover and freeze for several hours or overnight. Decorate the desert just before serving.