Measure the Chex cereal into a large mixing bowl. You don't need perfectly full cups, a little over is fine.
To a microwave-safe mixing bowl, add the butter, chocolate chips, and peanut butter. Microwave for 1 minute and then stir. Heat for another 30 seconds and stir. Repeat until smooth.
Pour in vanilla and stir to combine. Pour the chocolate-peanut butter mixture over the cereal.
Use a rubber spatula to stir cereal until it's completely coated in chocolate. Transfer the cereal to a gallon-sized ziploc bag.
Pour the powdered sugar into the bag and seal. (Double-check your seal, trust me.) Shake bag until cereal is completely coated in sugar.
Transfer the puppy chow to a mixing bowl and serve.
Notes
Store any leftovers in an airtight container for up to 1 week in the fridge or in the freezer for up to 4 months in an airtight container or freezer bag.