LINE 8-inch square pan with foil, with ends of foil extending over sides. Microwave 4 oz. chocolate, 6 Tbsp. butter and water in large microwaveable bowl on HIGH 2 min. or until butter is melted; stir. Add dry pudding mix; stir 2 min.
ADD powdered sugar, 1 cup at a time, stirring after each addition until well blended. Press onto bottom of prepared pan.
MICROWAVE remaining chocolate and butter in large microwaveable bowl 1-1/2 min. or until butter is melted. Stir until chocolate is completely melted. Spread over pudding mixture. Refrigerate 2 hours or until firm.