Pour the orange liqueur into a 1-quart measuring cup.

Grate the colored part of the skins of 3 oranges and the 1/2 lemon into the cup.

Strain in enough orange juice so liquid measures 2 cups.

Beat the egg yolks and granulated sugar in a 3-quart mixing bowl until the mixture is pale yellow and forms a ribbon.

Beat in the cornstarch and the orange juice mixture.

Pour the mixture into a 2 1/2-quart heavy-bottomed enameled saucepan.

Stir over moderate heat with a wooden spoon until the mixture heats through and thickens, but does not come to a simmer or a temperature of more than 170 degrees.

Ensure the mixture coats the spoon lightly.

Remove from heat and beat for a moment to stop the cooking process.

Beat the egg whites and salt in a separate bowl until soft peaks are formed.

Sprinkle in the 1 Tb of granulated sugar and beat until stiff peaks are formed.

Fold the egg whites into the hot orange mixture, and then fold over a bowl of ice until thoroughly chilled so that the custard will not separate.

Beat the chilled whipping cream until stiff peaks are formed and gently fold into the chilled mousse.

Turn the mousse into orange-shell cups, dessert cups, or a serving bowl.

Cover and freeze for several hours or overnight.

Decorate the dessert just before serving with glazed orange peel, angelica cut into leaf shapes, mint leaves, or whipped cream.