Grease the bottom and sides of a 13×9 pan with some butter.
Dump the bag of marshmallows into a large microwavable bowl (use a taller one, not wider one if possible because the marshmallows will expand and rise). Cut the butter into tablespoon chunks and add it to the marshmallows.
Microwave for 2 minutes, or until the marshmallows have puffed up and you can see them above the bowl.
Mix the marshmallows and return to the microwave for 1 minute and 30 seconds. The marshmallows should bubble.
Mix the marshmallows again and add the rice krispies. Stir well to combine.
Pour into greased 13×9 pan and pat down with the greasy butter wrapper, or the back of a greased spoon. Let sit to cool and set. Cut and enjoy!
Recipe Notes
Total Recipe Cost: $2.34
Optional: add some sprinkles to make this treat more festive!