Melt the butter and marshmallows in a large pot over medium-low heat, stirring constantly. Once completely melted and smooth, add the cereal and stir until completely combined. Evenly distribute the marshmallow mixture into the prepared pan and press it into an even layer. Allow it to set in refrigerator for at least 30 minutes before cutting. Remove the cooled treats from the pan to a cutting board. Using a 2 1/2-inch round biscuit cutter, cut out rounds and set aide.