Step 1: Measure the Chex cereal into a very large bowl or a large, sturdy resealable plastic bag. Set aside.
Step 2: In a microwave-safe bowl, combine the chocolate chips, peanut butter, and butter. Microwave on high for 60 seconds. Stir well. If not fully melted, microwave for an additional 30 seconds, then stir again until the mixture is completely smooth and creamy. Be careful not to overheat.
Step 3: Stir the vanilla extract into the melted chocolate-peanut butter mixture until well combined.
Step 4: Pour the warm chocolate mixture over the Chex cereal. Gently fold and stir with a rubber spatula until all the cereal is evenly coated. Alternatively, if using a resealable bag, pour the mixture in and gently shake and tumble to coat.
Step 5: Add the powdered sugar to the coated cereal. If using a bowl, sprinkle it over the top. If using a bag, pour it directly into the bag.
Step 6: Seal the bag (if using) and shake vigorously for 2-3 minutes, tumbling the cereal until every piece is generously coated in powdered sugar. If using a bowl, stir and toss with the spatula until thoroughly coated. This step is key for perfect Chex Mix Puppy Chow!
Step 7: Spread the Chex Mix Puppy Chow onto a baking sheet lined with parchment paper or wax paper in a single layer to cool and set for about 15-30 minutes. This allows the coating to firm up slightly.
Step 8: Once set, transfer your delicious Chex Mix Puppy Chow to an airtight container. Serve immediately or store for later enjoyment.