Preheat oven to 350°.
Mix together cake mix and melted butter. Reserve 1/2 cup of the mixture.
Pat cake mixture into bottom of ungreased 13x9 baking dish.
Mix together peanut butter and fluff. This gets a little messy and difficult. I used a spatula and mixed by hand, but you can also use a handheld mixer.
Spread peanut butter mixture on top of cake layer. Its easier to use your fingers to press and spread the fluff mixture. Make sure to spread out evenly.
Crumble the reserved 1/2 cup of devil food mixture all over fluff mixture.
Bake for 20 minutes in preheated oven.
Let cool completely. Cut into bars.
Enjoy!