Coat a 9x9 baking dish with cooking spray or line with parchment paper. Set aside.
In a large microwave-safe bowl, melt the butter and marshmallows starting with 90 seconds and then moving down to 20 second increments, stirring between each until marshmallows are smooth and fully melted.
Immediately stir with Fruit Loops, adding just 1 cup at a time and then tossing until all are added and uniformly covered.
Transfer into the prepared bowl, pressing down with greased fingers and another piece of parchment to make them snug.
Allow to cool, then remove from the baking pan and cut into 12 squares using a sharp knife.
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