Additional whipped cream and grated lime rind for garnish
Combine the sugar, cornstarch and salt in a saucepan or in the top part of a double boiler. Gradually stir in the water. Cook over hot, not boiling, water or over low direct heat until thickened, stirring constantly. Add the lime juice and cook until thickened.
Add a little of the hot mixture to the beaten egg and then stir it into the remaining hot mixture. Cook for one to two minutes over low heat, stirring constantly. Remove from the heat and add the grated lime rind and vanilla. Cool.
Fold in the whipped cream. Serve in sherbet glasses, garnished with additional whipped cream and a sprinkling of lime rind.