Start by greasing a 9x13 inch baking dish with a bit of butter or lining it with parchment paper for easy removal later.
In a medium saucepan over low heat, combine the butter and peanut butter. Stir continuously until both are melted and fully combined.
Once the butter and peanut butter are melted, add the mini marshmallows to the saucepan. Stir continuously until the marshmallows are completely melted and the mixture is smooth.
In a large mixing bowl, combine the chopped Nutter Butter cookies and Rice Krispie cereal. Pour the melted peanut butter and marshmallow mixture over the dry ingredients. Use a spatula to gently fold everything together until evenly coated.
Transfer the mixture into the prepared baking dish. Use the back of the spatula or your hands to press the mixture evenly into the dish.
Allow the bars to cool at room temperature for about 30 minutes, or until they are set. For faster results, you can place them in the refrigerator for about 15 minutes.
Once set, lift the bars out of the baking dish using the edges of the parchment paper. Cut into squares or rectangles, and enjoy!
