In 4-quart Dutch oven, melt 10 tablespoons butter over low heat. Add marshmallows; cook and stir until melted. Pour cereal over marshmallow mixture; stir together with large wooden spoon until all cereal is coated with marshmallow. Quickly press cereal mixture in pan with well-buttered hands. Use sheet of waxed or cooking parchment paper on top of cereal mixture to pack it down into the pan. Refrigerate cake until well chilled and set, about 2 hours.