1. Place the marshmallows and butter in a large, microwaveable bowl and heat in microwave on high for 1 1/2 to 2 minutes, or until marshmallows begin to expand.
2. Mix marshmallows and butter together, then add in Cheerios. Coat Cheerios with marshmallow mixture completely.
3. Line a 9 x 13 inch baking dish with wax paper and spread Cheerio Treats out evenly in the pan. Sprinkle with rainbow sprinkles.
4. Refrigerate for at least 30 minutes before serving. Enjoy!