Add the Chex cereal to a large bowl. Set aside.
In a small microwave safe bowl, microwave the white chocolate chips, peanut butter and butter in 30 second intervals, mixing until smooth. Mix in the vanilla, if using.
Pour the chocolate mixture on top of the Chex cereal. Using a rubber spatula, gently mix the chocolate mixture into the Chex, careful not to crush the cereal.
Add the powdered sugar to a large freezer bag.
Scoop the Chex mixture in the bag with the powdered sugar. Close the bag and shake until the Chex is fully coated in powdered sugar. You may need to press on the bag to separate clumps.
Line a baking sheet with parchment paper. Spread the Chex mixture evenly onto the prepared baking sheet. Allow the chocolate to harden, then store in an airtight container.