Heat oven to 350 degrees. Mix dry cake mix, butter and 1/3 c milk until blended; press 2/3 onto bottom of 13x9-inch pan. Bake 12-14 minutes or until center is almost set; cool 3 minutes.
Mix peanut butter and remaining milk; spread onto crust. Top with spoonfuls of marshmallow creme and remaining cake mixture; press gently into cake mixture.
Bake 18 minutes or just until center is set. Cool before cutting into bars.
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