PREHEAT oven to 325º F.
COMBINE crushed cookies and butter in medium bowl. Press onto bottom and up side of ungreased 9-inch tart pan with removable bottom. Bake for 15 minutes. Cool completely in pan on wire rack.
MICROWAVE chocolate and 1 cup cream in medium, uncovered, microwave-safe bowl on MEDIUM-HIGH (70%) power for 1 minute; STIR. If pieces retain some of their original shape, microwave at additional 10- to 15-second intervals, stirring just until smooth. Cool to room temperature.
BEAT remaining 1 cup cream and powdered sugar in chilled medium mixer bowl until stiff peaks form. Carefully fold whipped cream into chocolate mixture until blended. Spoon into prepared crust; smooth top.
REFRIGERATE for 4 hours or until firm.
NOTE: For a more sophisticated flavor, add two tablespoons orange-flavored liqueur into chocolate mixture before spooning into crust. Tart slices can also be garnished with whipped cream and fresh berries.