Pour the Chex cereal into a large bowl and set it aside.
In a microwave-proof bowl, combine the chocolate chips, peanut butter, and butter. Microwave for 30-90 seconds, 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, trying not to crush the cereal. Divide between two gallon-size Ziploc bags and pour the powdered sugar into one and the brownie mix into the other, then seal and shake to evenly coat all of the cereal.
Spread the cereal out on parchment paper to let the chocolate firm up a bit, for about 20 minutes.
In a large bowl, combine the chocolate coated cereal with the pretzel pieces, candy corn, Oreo pieces, and Reese’s pieces. Store in an air-tight container.
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