Step 1: Measure out 9 cups of rice square cereal into a very large mixing bowl and set aside.
Step 2: In a microwave-safe bowl, combine the chocolate chips, creamy peanut butter, and unsalted butter.
Step 3: Microwave the mixture in 30-second intervals, stirring well after each, until smooth and melted (usually 1-2 minutes total).
Step 4: Stir the vanilla extract into the melted chocolate mixture until fully combined.
Step 5: Pour the warm chocolate mixture over the cereal in the large bowl. Use a rubber spatula to gently fold the mixture until the cereal is evenly coated, being careful not to crush the squares.
Step 6: Place 1 cup of powdered sugar into a large gallon-sized plastic bag or a lidded container. Add the chocolate-coated cereal, then top with the remaining 1/2 cup of powdered sugar.
Step 7: Seal the bag or container and shake vigorously until all the cereal is thoroughly coated with powdered sugar.
Step 8: Spread the puppy chow onto a baking sheet lined with parchment paper to cool completely before serving or storing. Store in an airtight container at room temperature.