Prepare a 9" x 9" baking pan by lining it with parchment paper. Allow some of the parchment to hang over the sides of the pan, making a sling. Spray the parchment and pan with cooking spray. 

Open the bag of M&Ms and measure out 2 tablespoons of candies. Set aside in a separate dish.

In a large bowl, add the Rice Krispies cereal. Set this aside too.

In a 2 quart saucepan, melt the butter and the marshmallows over medium-low heat. Stir constantly until all the ingredients are melted completely and smooth.

Once melted, turn off the burner, and immediately pour the marshmallow mixture over the cereal. 

Using a rubber spatula, quickly mix the marshmallow mixture into the cereal until completely coated. 

Add all but the reserved 2 tablespoons of M&Ms to the marshmallow cereal mixture and quickly give a final stir until the candy is well distributed.

Dump the mixture into your prepared baking dish. Press the cereal mixture into the dish evenly and compact. You can do this cleanly by spraying a piece of plastic wrap with cooking spray and setting it over the Rice Krispies mixture while you're pressing.

Once the mixture has been pressed into the pan, sprinkle the remaining 2 tablespoons of M&Ms over the top and press into the treats to adhere. 

Allow the Rice Krispie treats to sit for 2 hours to set up.

Once the treats are completely set, remove them from the pan using the parchment paper sling. Discard the parchment and trim the edges of the Rice Krispie Treats to make an even square and then cut into twelve portions.

In the microwave (or in a heatproof bowl set over barely simmering water), melt the white chocolate or candy coating.

Using a piping bag or a spoon, drizzle the melted candy coating over each of the treats. Decorate with Halloween themed sprinkles and enjoy once the white candy/chocolate drizzle has set completely!