In a second large microwave-safe bowl, combine the peanut butter, chocolate chips, and butter. Microwave in bursts of 15 seconds, stirring in between each burst for 15 seconds until the mixture is melted and smooth. Add in the vanilla and salt. Pour over the cereal. Stir well, but gently, until the cereal is evenly coated.
Pour half of the cereal mixture into a gallon-sized sealable plastic bag and pour half of the powdered sugar over it. Seal and shake until the cereal is evenly coated. Repeat with the other half of the cereal and powdered sugar in the same or a different bag. Shake until all the cereal is well coated! (Separating the process into two batches makes it easier and allows all the cereal to get evenly coated.)
Remove into a large bowl and if desired, stir in the M&M's, Reese's Pieces or any other additions desired. Enjoy!
Store leftover Puppy Chow in an airtight container at room temperature.
Nutritional information does not include the optional candy mix-ins.