Into a 9×13 baking pan spread a piece of parchment paper so that the edges hang over the sides of the pan, and then spray with nonstick spray.
Pour Rice Krispies into a large bowl and set aside.
In a medium saucepan, melt butter over low heat. Add 4 cups mini marshmallows and heat, stirring frequently, until melted and mostly combined
Turn off heat, add peanut butter and stir to combine.
Immediately pour peanut butter mixture into the bowl of Rice Krispies, add Fluff and remaining 2 cups mini marshmallows and stir to coat and combine.
Turn out into prepared baking dish and press down into an even layer.
Let set for at least 15 minutes, and then slice into 12 large bars. Serve and enjoy!