HEAT oven to 350°F.
COMBINE dry cake mix, butter and 1/3 cup milk; press 2/3 onto bottom of 13x9-inch pan. Bake 12 to 14 min. or until center is almost set; cool 3 min.
MIX peanut butter and remaining milk until blended; spread onto crust. Top with small spoonfuls of the marshmallow creme and remaining cake mixture. Sprinkle with nuts and chocolate; press gently into cake mixture.
BAKE 18 min. or just until center is set. Cool completely before cutting into bars.