Grease a 9x13 inch baking dish with cooking spray and set it aside.
In a large pot, melt the butter over low heat.
Once the butter has melted, add the marshmallows to the pot and stir constantly until they have completely melted.
Remove the pot from the heat and stir in the Rice Krispies cereal until it is well coated with the marshmallow mixture.
Pour the mixture into the prepared baking dish and use a spatula to press it down evenly.
Let the Rice Krispie Treats cool completely, then cut them into squares.
Enjoy your delicious and easy-to-make Rice Krispie Treats!