In large saucepan mix gelatin, 1/4 cup sugar, the salt, milk and egg yolks; blend well. Stir over low heat until gelatin is dissolved and mixture coats metal spoon (175° on candy thermometer). Remove from heat. Stir in vanilla. Chill until mixture mounds slightly when dropped from spoon, stirring occasionally. Peel mango. Cut flesh off seed in slices, scraping off any flesh clinging to seed. Reserving prettiest slices for garnish, puree enough mango (about 1-1/4 cups) in blender or food processor to yield 1 cup. Mix in lime juice. Stir into chilled custard. In a small mixer bowl, beat egg whites until foamy. Gradually beat in remaining 1/4 cup sugar until soft peaks form. Fold egg whites and whipped cream into mango mixture. Turn into 6-cup ring or other mold rinsed with cold water. Chill until firm. Unmold. Garnish with reserved mango slices.