Lightly spray a 9 by 9 by 2-inch baking pan with vegetable spray; set aside. Melt butter in a large pot over medium heat. Add salt and marshmallows. Stir with a wooden spoon until melted. Remove from heat. Add the cereals and stir to combine.
Transfer mixture to prepared pan. Coat hands with vegetable spray, pressing mixture evenly into pan. Let cool, about 30 minutes.
Cut into 3 by-3-inch bars. Store in an airtight container for up to 3 days.
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