Melt the butter in a large stock pot over medium low heat.

When the butter is melted, add the marshmallows and stir to mix. Continue stirring until the marshmallows are fully melted.

Remove the pan from heat and stir in the cereal until the cereal is fully coated with marshmallow.

Line a 9×13-inch baking dish with parchment paper and pour the cereal mixture into the pan.

Lightly press the cereal into an even layer in the pan and allow it to cool completely, at least one hour.

When the bars are cool, pull the edges of the parchment paper up to remove them from the pan and transfer them to a flat surface. Cut into 20 even bars.