PREP: Preheat oven to 350 degrees F. Line a 15x21-inch baking sheet (or 2 smaller baking sheets) with foil, parchment paper, or a Silpat liner and generously grease with nonstick cooking spray.
CEREAL: Add the 8 cups of Rice Chex cereal to the lined tray and spread in one even layer.
SYRUP: In a heavy medium-sized pot, add the 1/2 cup butter (cut into individual tablespoons), 1 cup packed light brown sugar, and 1/4 cup corn syrup. Place over medium to medium-high heat and stir constantly. Continue to stir until the mixture comes to a rapid boil. Boil for exactly 1 minute, stirring constantly, and remove from heat. Add in the 1/4 teaspoon baking soda and 1/4 teaspoon fine sea salt. Add in the vanilla extract and stir to combine. Pour over the cereal on the sheet pan and, working quickly, stir until cereal is evenly coated in the syrup mixture.
SUGAR: Sprinkle the white granulated sugar evenly over the syrup coated cereal. (Don't toss to coat)
BAKE: Bake for 5 minutes, remove from the oven, and toss using a metal spatula. Spread mixture again to one even layer. Return to the oven and bake for 4 more minutes. Remove from the oven and toss again. Spread in one even layer and let cool for 15 minutes.
COAT IN POWDERED SUGAR: Transfer snack mix into a 2-gallon sized zipper-top bag. Add 1/2 cup powdered sugar on top. Seal the bag (do not squeeze air out of the bag) and then shake to coat the cereal in the mix. It will seem like it's not enough powdered sugar, but keep shaking; it will coat the cereal. Once the cereal is generously coated, pour it back onto the sheet pan in a single layer. Sprinkle the sugar cookie M&M's evenly over the coated cereal.
WHITECHOCOLATE: In a medium-sized microwave-safe bowl, combine the white chocolate chips and 1 teaspoon coconut oil. Melt, in bursts of 15 seconds, stirring in between each burst for 15 seconds, or until white chocolate is fully melted and smooth. Transfer melted chocolate to a small plastic bag and seal without any air in it. Cut off the very tip with scissors and pipe evenly over the snack mix until all the mix is coated. Immediately add the sprinkles if using. Let stand for 30 minutes to an hour to harden.
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Notes
Storage: Store in an airtight container at room temperature for up to a week. We like it best when used within 3-4 days. You can freeze it in an airtight container/bag for up to 3 months; thaw at room temperature. There is a slight flavor and texture loss after freezing this Sugar Cookie Snack Mix.