In a microwave-proof bowl, combine the chocolate chips, peanut butter, and butter. Microwave for 30-120 seconds at a time, stirring every 30 seconds until the chocolate chips are completely melted and smooth. Stir in the vanilla.
Pour the chocolate mixture over the Chex cereal and gently stir to coat, avoiding crushing the cereal.
Pour the powdered sugar into a gallon-sized Ziploc and the brownie mix into another gallon-sized Ziploc bag, and divide the chocolate-covered Chex cereal into the baggies. Seal the bags, leaving plenty of air inside.
Shake the baggies to coat the Chex cereal evenly.
Spread the cereal out on parchment paper to let the chocolate firm up a bit, for about 20 minutes in the freezer.
In a large bowl, combine the chocolate-coated cereal with pretzel balls, candy corn, Oreo pieces, Reese's Pieces, and Halloween sprinkles. Store in an airtight container.