Grease a 9x13 inch baking dish with cooking spray. In a pot, melt the butter over low heat. Add the mini marshmallows and peeps to the pot and stir until melted.
Add the Rice Krispies cereal into a large bowl. Remove the pot from the heat and pour marshmallows into cereal. Immediately stir until the cereal until it is fully coated in the marshmallow mixture.
Pour the mixture into the greased baking dish and press it down firmly with a spatula or your hands. Let it cool and set for at least 30 minutes before cutting into squares and serving.
** If you wanted to make a rainbow layered treat, make multiple batches with different colored peeps. Press in first layer, then layer second on top of that and press down. In order to make it more than 3 and still fit in the pan cut this recipe in half so layers aren't as thick.