Stir in 12 oz white baking chips and cook in 20 second bursts until they stir smooth.
Add 2 Tablespoons lemon juice and zest from 1 lemon, stirring to combine. It may seize a little bit. It should still be able to coat the cereal, it doesn't have to be perfectly smooth. If it is really thick, you can stir in a splash of cream to loosen it up again.
Fold in 8 cups rice or corn Chex cereal until coated.
Spoon cereal into a gallon zipper bag. Add the 1½ cups powdered sugar, seal the bag, then shake until the cereal is coated.
Spread the puppy chow out to cool.
Once completely set, put in an airtight container for storage.