COAT 13 x 9-inch baking pan and rubber spatula with nonstick cooking spray.
MELT margarine over low heat in medium, heavy-duty saucepan. Add marshmallows and peanut butter; stir until melted. Remove from heat. Add cereal; stir until coated. Press onto bottom of prepared baking using prepared rubber spatula.
COMBINE margarine, evaporated milk, sugar and salt in medium, heavy-duty saucepan. Bring to a full rolling boil over medium heat, stirring constantly. Boil, stirring constantly, for 4 to 5 minutes. Remove from heat.
ADD marshmallows, morsels, peanut butter and vanilla extract; stir vigorously for 1 minute or until marshmallows and morsels are completely melted. Pour over crust. Sprinkle nuts over fudge; press in slightly. Refrigerate until set. Cut into squares.