Line 8-inch square pan with foil, with ends of foil extending over sides. Microwave half the chocolate, 6 Tbsp. butter and water in large microwaveable bowl on HIGH 2 min. or until butter is melted; stir with whisk until chocolate is completely melted and mixture is well blended.
Add dry pudding mix; stir 2 min. Add sugar, 1 cup at a time, stirring after each addition until 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 and mixture is well blended. Spread over fudge layer in pan; sprinkle with marshmallows and nuts. Refrigerate 2 hours or until firm. Use foil handles to lift fudge from pan before cutting into squares.