Measure the cereal into a very large mixing bowl. In a large 2-gallon zip-top bag, combine the dry pistachio pudding mix and powdered sugar. Shake lightly to blend and set aside.

In a microwave-safe bowl, combine white chocolate chips and coconut oil. Microwave at 50% power in 30-second intervals, stirring vigorously after each, until completely melted and smooth.

Pour the melted white chocolate over the cereal. Use a large spatula to gently fold and stir until all the cereal pieces are evenly coated.

Gently fold in the chopped pistachios so they adhere to the wet chocolate on the cereal.

Carefully transfer the sticky cereal mixture into the zip-top bag with the sugar and pudding mix. Seal the bag tightly, leaving air inside, and shake vigorously until all pieces are completely coated.

Spread the coated puppy chow onto a parchment-lined baking sheet to cool and set for 30 minutes before serving or storing.