In a large bowl, mix together the Rice Krispies and one cup of the mini marshmallows.
In a large saucepan over low heat, melt the butter.
Add the remaining 7 cups of mini marshmallows and stir into the melted butter. Continue stirring over low heat until the marshmallows are completely melted into the butter.
Pour the melted butter and marshmallow mixture over the Rice Krispies cereal mixture and gently stir with a rubber spatula until evenly combined.
Spread the rice krispie treat mixture into a 9-inch square pan. Spray the back of a large metal spoon with cooking spray or lightly oil it with vegetable oil to prevent sticking. Use the back of the spoon to spread the mixture evenly into the pan, pressing it into the corners and smoothing the top.
Pop the pan into the refrigerator to allow the rice krispie treats to cool for at least 20 minutes before slicing and serving.