Line a large baking sheet with parchment and set aside. Measure the cereal into an extra-large bowl (or divide between two bowls if needed).
In a medium microwave-safe bowl, combine white chocolate chips, peanut butter and coconut oil. Microwave on medium power in 20–25 second bursts, stirring between bursts, until smooth and glossy (about 1 to 1 1/2 minutes total). Stir in vanilla extract off the heat.
Sprinkle the protein powder and pumpkin pie spice over the cereal and gently toss with a large spatula until the cereal is evenly coated with the dry mix.
Pour the melted white chocolate–peanut butter mixture over the coated cereal. Fold carefully with a spatula or large spoon so the cereal is lightly and evenly coated without crushing the pieces.
Working in batches if necessary, transfer coated cereal into a large sealable plastic bag. Add about half of the powdered sugar, seal, and shake vigorously until pieces are evenly dusted. Repeat with remaining powdered sugar if any spots look sticky.
Pour the puppy chow onto the prepared parchment in a single layer to cool and set for 10–15 minutes. Once set, break up any clumps with your hands or a spatula and transfer to a serving bowl.
Optional: sprinkle a little extra pumpkin pie spice over the top and toss lightly before serving. Store leftovers in an airtight container at room temperature for up to 4 days.