In a large mixing bowl, add 8 cups of Rice Chex cereal. Set aside.
In a microwave-safe bowl, combine semi-sweet chocolate chips, creamy peanut butter, and unsalted butter. Microwave on high for 1 minute; Stir and microwave for another 30 seconds until the chocolate mixture is smooth.
Add the Chex cereal to the melted chocolate mixture, stirring gently to coat the cereal evenly. I add it a little at a time to help make the mixing more manageable.
Pour the Chex into a large Ziploc bag, and add the powdered sugar. Then, seal the bag and shake until all the cereal is covered in powdered sugar.
Spread the coated cereal mix onto a baking sheet lined with parchment paper to cool. Once at room temperature, mix in the Halloween M&M's, mini Reese's Peanut Butter Cups, candy corn, pretzel twists, and Halloween sprinkles.
Enjoy this festive Halloween treat with your family, or package it into individual baggies for a fun Halloween party snack!