Prepare a 9” x 13” glass pan by lightly spraying it with cooking spray (add parchment paper if desired).
In a large microwave-safe bowl, add the marshmallows and butter. Microwave for 2-3 minutes total in 1-minute increments.
Once completely melted, remove the marshmallow mixture from the microwave and stir briefly to combine. Immediately mix in the salt and vanilla extract.
Quickly add in the rice krispie cereal and mix well. Then, mix in the M&Ms until they are evenly distributed throughout the mixture.
Evenly press the rice krispie mixture into the prepared pan with a rubber spatula. Cover and let them sit for about an hour to fully set up. Once they solidify, cut into squares and serve.
Notes
Be sure to use a large bowl to heat the marshmallows and butter to avoid overflow. The marshmallows will expand quite a bit in the microwave.
Have all of the ingredients prepped and ready to go before microwaving the marshmallow mixture. They need to be added immediately after removing the mixture from the microwave before it cools and solidifies.
The more pressure you apply to press the rice krispie mixture into the pan, the denser your rice krispie treats will be.
Cover and place the rice krispie treat pan into the refrigerator or freezer to set up faster.