Line a 9×13 inch pan with parchment paper. Lightly grease the parchment.
Melt the butter in a large pot over low heat. Add the marshmallows and stir constantly until completely melted and smooth.
Remove from heat. Immediately add the rice cereal and stir quickly until evenly coated.
Transfer the mixture to the prepared pan. Press evenly using a greased spatula or wax paper — do not pack too tightly.
Let cool completely at room temperature for at least 30 minutes before cutting into 16 squares.
Build your own version of this recipe and Nutrifax generates a print-ready Nutrition Facts label — free.