Lightly spray your pyrex glass container with olive oil cooking spray and set it aside.
Measure out 4 cups of marshmallows and place them in a small bowl and then also measure out 6 cups of rice krispies cereal and place them in a large bowl, so you are able to quickly add both ingredients when the butter melts.
Cut up the 4 Tablespoons butter into smaller sized squares, so it melts quicker and place it in a large ceramic saucepan on medium heat. Stir the butter constantly, allowing it to melt fast, but does not burn.
When the butter melts, immediately add the marshmallows and stir them constantly using a wooden spoon until they lose their shape. This process should only take a couple minutes.
Remove the saucepan from heat and mix in the rice krispies cereal, coating the cereal in the butter marshmallow syrup mixture.
Pour the coated rice krispie treats in your pre-sprayed Pyrex container and use a flat spatula to spread it carefully spread out the cooked rice krispies treats evenly within the glass container.
Let them cool for 15-20 minutes before cutting them into squares. I find it's easier to cut up the rice krispies treats by flipping the Pyrex pan upside down on parchment paper.
Enjoy immediately or store your uneaten rice krispies treats in the Pyrex with plastic wrap covering to keep them moist.
Preparation is key to this quick and easy recipe, which is much easier to make if you pre-measure out the cereal and marshmallows into bowls allowing you to pour them both into the saucepan.