1. Prepare TART SHELL.
2. Place chocolate chips, 1/4 cup milk and sugar in small microwave-safe bowl. Microwave at MEDIUM (50%) 1 minute or until hot. Stir or whisk until smooth; cool to room temperature (about 20 minutes).
3. Sprinkle gelatin over cold water in small cup; let stand 2 minutes to soften. Place white chips and remaining 1/4 cup milk in second small microwave-safe bowl. Microwave at MEDIUM 1 minute or until hot; stir or whisk until smooth. Add gelatin mixture and vanilla; stir until gelatin is dissolved. Cool to room temperature (about 20 minutes).
4. Beat whipping cream in small bowl until stiff; fold 1 cup whipped cream into vanilla mixture. Fold chocolate mixture into remaining whipped cream. Alternately spoon chocolate and vanilla mixtures into baked tart shell; with spatula, swirl for marbled effect. Refrigerate until firm. 8 servings.
TART SHELL: Heat oven to 375°F. Grease bottom and sides of fluted 8 or 9-inch tart pan. Beat 1/2 cup (1 stick) softened butter or margarine and 2 tablespoons sugar in small bowl until blended. Add 2 egg yolks; mix well. Stir in 1 cup all-purpose flour until mixture is crumbly. Press onto bottom and up sides of prepared pan. (If dough is sticky, sprinkle with 1 tablespoon flour.) Prick bottom with fork to prevent puffing. Bake 8 to 10 minutes or until lightly browned. Cool completely.