Pour the cereal into a very large mixing bowl. Set it aside.

In a medium-sized saucepan over low heat, combine the semi-sweet chocolate chips, creamy peanut butter, and unsalted butter. Stir continuously as the ingredients begin to melt. Continue to stir until the mixture is smooth and fully melted, ensuring that it doesn't burn or stick to the bottom of the pan. Once the mixture is fully melted and smooth, remove it from the heat.

Pour the melted chocolate mixture over the Chex cereal in the large mixing bowl. Gently and carefully fold the chocolate mixture into the cereal until all the cereal pieces are evenly coated. Be cautious not to crush the cereal pieces.

In a large paper bag or a large zip-top plastic bag, add half of the powdered sugar. Carefully transfer half of the chocolate-coated Chex cereal into the bag with powdered sugar. Seal the bag and shake it vigorously to coat the cereal evenly with powdered sugar. Ensure that every piece of cereal is coated. Repeat with the other half of the ingredients. Adding more powdered sugar as needed to fully coat the cereal.

After coating with powdered sugar, spread the puppy chow out on a baking sheet or wax paper to allow it to cool and set. This should take about 15-30 minutes.

Once the puppy chow has cooled and the chocolate has set, it's ready to enjoy. Store any leftovers in an airtight container.