In large pot over low heat, melt the butter. Once it's melted, add the marshmallows and stir often until it's smooth and all lumps are gone. Remove from heat.
Finally, add in the Rice Krispies and mix until combined. Spray a 9×13 pan with cooking oil and the back of your spoon. Transfer the mixture to the pan and press into it with the back of the spoon or use your hands if it's sticking too much. Let set for 1-2 hours.