1. Melt chocolate with milk in small saucepan over very low heat, stirring constantly, until smooth. Add marshmallows; stir until smooth. Remove from heat; cool completely.
2. Beat 2/3 cup whipping cream until stiff in small bowl; fold into chocolate mixture. Divide evenly into crusts. Cover; refrigerate until firm. Just before serving, beat remaining 1/3 cup whipping cream, powdered sugar and vanilla until stiff; serve on tarts as garnish. Refrigerate leftovers. 8 servings.