HEAT oven to 350°F. Mix dry cake mix, butter and 1/3 cup milk until blended; press 2/3 onto bottom of 13x9-inch pan. Bake 12 to 14 min. or until center is almost set; cool 3 min. Meanwhile, mix peanut butter and remaining milk.

SPREAD peanut butter mixture 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.