Melt the chocolate mixture: In a microwave-safe bowl, combine the dark chocolate chips, peanut butter, and butter. Microwave on high for 30-second increments, stirring after each, until the mixture is smooth. Stir in the vanilla extract.

Mix with cereal: Place the Chex cereal in a large bowl. Pour the melted chocolate mixture over the cereal and gently stir until all pieces are evenly coated.

Coat with powdered sugar: Transfer the coated cereal into a large ziplock bag. Add the powdered sugar, seal the bag, and shake until the cereal is well-coated with powdered sugar.

Cool and store: Spread the coated cereal onto a baking sheet to cool and set. Once cooled, store in an airtight container or package for your market.