Line 8-inch square pan with foil, with ends of foil extending over sides. Microwave semi-sweet chocolate and milk in microwaveable bowl on HIGH 2 min. or until chocolate is almost melted, stirring after 1 min. Stir until chocolate is completely melted and mixture is well blended. Stir in nuts. Pour into prepared pan; spread to evenly cover bottom of pan.
Microwave white chocolate in separate microwaveable bowl 1-1/2 min. or until chocolate is almost melted, stirring after 1 min. Stir until completely melted. Add peanut butter; stir until melted. Spread over semi-sweet chocolate layer in pan.
Refrigerate 2 hours or until firm. Use foil handles to remove fudge from pan before cutting into pieces.