-rhubarb Garnish: a handful of strawberries, blackberries, or raspberries Cook the sugar and water until it forms a thick syrup (the temperature should be 238 degrees). Beat the egg whites and when they foam, add the salt and cream of tartar, and continue to beat until they start to form soft peaks. Add in a slow steady stream the hot sugar syrup, beating constantly, and continue to beat until the mixture has cooled and is thick. Beat the cream in a separate bowl until almost stiff, then beat in the liqueur. Fold the whipped cream into the egg white-sugar syrup mixture and freeze for 2 hours. Remove the bowl from the freezer and stir in the pureed rhubarb. If you are not serving immediately, hold the mousse in the freezer. Serve in a glass bowl or in sherbet glasses, decorated with berries. Serves 8. [ The L. L. Bean Book of NEW New England Cookery ]