Arrange a 9x13 inch baking pan and line it with parchment paper or lightly spray with non-stick spray to prevent sticking.
Measure out the marshmallows and cereal; set aside for easy addition during cooking.
4 cups small marshmallows
Place the butter in a large saucepan over medium heat, and stir until melted with a shiny, bubbling surface for about 2 minutes.
4 cups small marshmallows
Add the marshmallows to the melted butter and continuously stir with a rubber spatula until the marshmallows are completely melted and the mixture is smooth, about 3-4 minutes. You will notice the mixture becoming glossy and slightly thickened.
4 cups small marshmallows
Remove the pan from heat and immediately add the crispy rice cereal. Gently fold and stir with the spatula until the cereal is evenly coated with the marshmallow mixture, which should be thick and glossy.
4 cups small marshmallows
Transfer the mixture into the prepared pan and press down firmly with a spatula or clean hands to evenly spread and compact it into the pan. Once pressed, let it set at room temperature for at least 15 minutes until firm and completely cooled.
Cut into squares or bars once cooled and set, then serve.
