Add white chocolate to a microwave safe bowl, and microwave for 30 second intervals until the chocolate is fully melted, stirring between each interval.
Combine the pistachio butter and melted white chocolate together, and stir until combined.
Add Chex cereal to a large bowl, and use a rubber spatula to scoop the pistachio-white chocolate mixture over the top of the cereal. Stir until the cereal is fully coated.
Add salted and roasted shelled pistachios to the coated cereal, along with the Brown Butter Seasoning, and mix until fully combined and coated.
Pour the entire mixture into a gallon size freezer bag, and add in 1/3 cup of the powdered sugar. Seal the bag closed, leaving some air in the bag, and shake until the mixture is fully coated. Add more powdered sugar, 1/3 cup at a time, and repeat until the puppy chow is coated to your liking.
Pour the White Chocolate Pistachio Puppy Chow into a separate clean bowl, and add your preferred add-ins. Mix to combine, and enjoy!
Store the Puppy Chow in an airtight container at room temperature for up to one week (or in the freezer for up to 2 months).