Spray a 9x13 pan with non-stick spray. Set aside.
In an extra large bowl, measure out the 8 cups of Rice Krispie Cereal. Set aside.
In a separate large microwave safe bowl, microwave the butter until completely melted. (about 45 seconds)
Add one entire bag of marshmallows, plus 1/3 of the second bag of marshmallows to the bowl with melted butter. Stir to coat the marshmallows in the melted butter.
Microwave for 1 minute, stirring at the 30 seconds mark.
Add the vanilla to the bowl and stir until well combined and smooth.
Pour the melted marshmallow mixture into the bowl with the Rice Krispie Cereal. Stir until well combined.
Fold in the remaining 2/3 bag of marshmallows.
Pour the Rice Krispie treat mixture into the prepared 9x13 pan.
Wet your hands with water and then gently press the Rice Krispie Treats into the pan.