In a large pot, melt butter over medium heat. Add marshmallows and turn heat to low. Stir until completely smooth. Remove from heat and quickly stir in peanut butter. Add cereal.
Fold in the mini marshmallows. Pour Krispies treats into a 9inch square baking dish lined with parchment paper (or buttered foil). DO NOT PRESS them in tightly. Just very gently even the top of the treats (I use the butter wrapper or marshmallow bag wrapper to press it in). Spread melted peanut butter morsels over the top.
Top the melted peanut butter morsels with the peanut butter cups. Press those in so they stick! Allow to set, about one hour. Cut into squares and store in an airtight container. ENJOY.
Notes
Line your baking dish! Rice Krispie Treats are STICKY! Place a square of parchment paper on the bottom of your baking dish to prevent sticking. The parchment makes it easier to lift the finished treats from the pan, too!
Use the butter wrapper or the plastic marshmallow bag to press the rice cereal mixture into the pan to keep your fingers from getting sticky.
Press LIGHTLY. When you add the treat mixture to the pan, use as gentle of a touch as possible. The harder you press, the harder and denser your finished Fluffernutter Rice Krispie Treats will be.
Store these bars in an airtight container at room temperature. Enjoy within 5 days for best taste--but I bet you'll eat them up long before then!