HEAT oven to 350°F.
MIX dry cake mix, margarine and 1/3 cup milk until blended; press 2/3 onto bottom of 13x9-inch pan sprayed with cooking spray. Bake 12 to 14 min. or until centre is almost set; cool 3 min.
COMBINE peanut butter and remaining milk; spread onto crust. Top with spoonfuls of marshmallow creme and remaining cake mix mixture. Sprinkle with chocolate and nuts; press gently into cake mix layer.
BAKE 18 min. or just until centre is set. Cool completely before cutting into bars.
serving size = 1 bar (28 g)