In a large pot on medium heat, add the butter. Let melt, stirring with a wooden spoon to help it melt.
Once melted and the butter started to bubble and get frothy, add the bag of marshmallows. Stir and let marshmallows melt.
Once the marshmallows have melted and mixed in with the butter, turn off heat.
Add the Rice Krispies and stir in a folding motion to mix together.
Once fully incorporated, scoop into a non-stick casserole dish. If you are using glass, spray the dish first with cooking spray to prevent sticking.
Using a spatula, press the Rice Krispie mix to flatten and evenly spread in the dish.
Let cool. Once cooled remove from the casserole dish and cut into square. If you're not using a non-stick dish, you can cut the Rice Krispies into squares in the dish.
Notes
I used a large casserole dish (11"x17") but the standard 9"x13" is fine.