Line an 8"-x-8" pan with greased parchment paper or butter it generously. (see note.)

In a large pot over medium-low heat, melt butter. Add the marshmallows. Stir until the marshmallows are melted and creamy. Remove the pot from the heat. Add the cereal. Stir until it's well coated.

Spoon the mixture into the prepared pan. Gently press mixture into pan with a small piece of greased parchment paper. Don’t press too hard. You won’t want to compact the cereal.

Let cool completely at room temperature for about two hours or chill in the refrigerator for 30 minutes. Cut into squares.