In a large microwave safe bowl, add marshmallows and butter and microwave for 1 minute or until marshmallows are soft and gooey.
Add rice crispies and stir until everything is mixed together.
Add to a greased 9 x 13 inch pan and press into the pan with your hands to smooth the tops and slightly compact. Use a little butter on your hands to prevent the cereal from sticking to them.
Let the mixture cool and set before slicing into squares and serving.