Lightly butter a 13 x 9- inch baking dish. Set aside.
In a Dutch oven melt butter over medium low heat until melted. Add marshmallows, turn heat to medium and stir marshmallows and butter together until marshmallows are melted. Stir until creamy.
Remove from heat; stir in cereal. Stir until cereal is well coated. Dump contents into prepared dish. With a buttered spatula press cereal mixture down.