Melt the butter and brown sugar over medium heat. Add in the marshmallows and stir constantly to prevent sticking or burning. Once completely melted pour over cereal. Quickly stir together and pour into prepared greased pan. Allow to cool and cut into squares. Store in an air tight container.
Instead of using a greased pan, I like to line mine with parchment paper. So much easier! You will want to grease the spoon though.