In a large pot over medium heat, melt butter completely, then add the marshmallows and stir until the marshmallows are melted and the butter is fully incorporated.
Turn off the heat, add the rice cereal, and stir until combined.
Line a baking sheet with parchment paper and grease with nonstick spray.
Pour the crispie mixture into the pan and spread to the edges. Use a rolling pin to gently smooth out the crispies in an even layer. Cover with plastic wrap and refrigerate for at least 30 minutes, until completely cooled and set.
Cut into bite-size pieces, or decorate with sugar sprinkles and create a sculpture with crispies!