Line a 9-inch × 13-inch baking sheet with parchment paper and then coat with non-stick spray. Set aside.
Premeasure all of your ingredients. You will need to work quickly with this recipe, so it really helps having everything ready to go.
In a large saucepan, melt the butter, salt, and marshmallows together until smooth.
Remove the pan from heat and stir in the vanilla and cereal until fully coated.
Gently and quickly fold in a 1/2 cup of the M&Ms. (If you stir to much, the colors will bleed and give the treats a grey color.) Transfer the cereal mixture to the prepared baking dish.
Use parchment paper or greased hands to gently press the mixture into the pan. You want an even thickness and for the cereal mixture to be pressed out to the edges fo the pan.
Sprinkle the remaining M&Ms over the top as well as some sprinkles and gently press them into the treats.
Allow the treats to set up for 30 minutes before removing from the pan and cutting into squares.
Microwave the candy melts in separate bowls (one for each color) in 30-second intervals, stirring between each or until fully melted. Drizzle over the top of the rice krispie treats. This step can be done before or after the initial cooling. Make sure the candy drizzle has cooled and hardened before consuming.
Notes
If you're using regular jet-puffed marshmallows instead of mini marshmallows, you'll need one 16-ounce bag.
You can substitute 1/2 cup of salted butter in place of the unsalted butter and salt.
You can cut your treats any size you like. I usually go for medium-sized squares and get 15 treats from one pan.
These treats will keep in an airtight container in a cool, dry place for up to 2 weeks.