MIX Base Cake and butter until well blended. Press firmly onto bottom of 2 lined full sheet pans (or 1 half sheet pan for trial recipe). Refrigerate until firm.
PREPARE pudding mix with milk in large bowl of electric mixer fitted with whip attachment as directed on package.
ADD whipped topping; beat on medium speed 30 seconds. Gently stir in OREO Pieces. Spread evenly over prepared crusts.
FREEZE several hours or overnight. Cut into 4-inch squares.