In a microwave-safe bowl, melt the butter on high for about 30 seconds until fully melted.

Add the mini marshmallows to the melted butter and microwave for an additional 1-2 minutes, stirring halfway through, until the marshmallows are completely melted and smooth.

Remove the bowl from the microwave and stir in the Fruity Pebbles cereal until fully coated with the marshmallow mixture.

Grease a 9x9 inch baking dish with butter or cooking spray, then pour the mixture into the dish, pressing it down evenly with a spatula.

Allow the treats to cool for about 30 minutes before cutting into squares and serving.