In a small bowl or a ramekin, melt the butter in the microwave for around 20 seconds, or until completely melted.
Place the marshmallows in the bowl, and microwave for an additional minute. Keep an eye on them! It will puff up, but will deflate as soon as you take it out of the microwave.
Mix together the marshmallows and the melted butter until smooth.
Mix in the Rice Krispies cereal until fully combined. Press into the bowl with a spatula and allow to cool.