Grease a 9"x12" baking pan with butter (generously, salted butter if you choose)
In a stock pan or Dutch oven, slowly melt the remaining butter and cook it until it reaches brown butter stage.
Add the marshmallows to the pan and stir constantly, scraping the bottom of the pan to dislodge any stuck-on and caramelized bits. Cook until the marshmallows are completely melted and then cook a little bit more, until it starts to turn light brown.
Add the cereal all at once and stir vigorously until it is mostly coated with the cooked marshmallow cream.