Prepare your baking dish by lining a 9x13-inch baking dish with parchment paper for easy removal of the bars.
In a medium saucepan over low heat, melt the butter. Once melted, add the mini marshmallows, stirring continuously until smooth, about 5 minutes.
Remove from heat and stir in the peanut butter until fully combined and smooth.
In a large mixing bowl, combine the Rice Krispie cereal and chopped Nutter Butter cookies. Pour the melted mixture over the cereal and cookies, folding gently until coated.
Transfer the mixture to the prepared baking dish and press down firmly and evenly.
Let the bars cool at room temperature for about 30 minutes to an hour or refrigerate to speed up the process.
Once set, lift the bars out using the parchment paper, cut into squares or rectangles, and enjoy!