Combine cornstarch, sugar and salt in the top part of a double boiler or in a saucepan. Heat one and three-quarters cups milk. Gradually add it to the cornstarch mixture and stir and cook over medium heat until mixture is very thick.
Blend remaining milk with egg yolks and add. Cook over hot water or low heat until mixture is almost as thick as mayonnaise. Remove from heat and stir in vanilla. Cool completely.
Spoon filling into the baked tart shells. Chill for three to four hours, or overnight. Just before serving arrange the grapes over the tops of the tarts. Garnish each tart with a tiny mound of whipped cream and a sprig of fresh mint.