In a medium glass bowl, microwave the chocolate chips, peanut butter and butter until melted and smooth- start with 1 minute, then stir, then microwave an additional 30 seconds. You can also do this on the stove- just warm it up in a small pan and stir over low heat until melted.
Mix the vanilla into the chocolate mixture, then pour the chocolate mixture over the Chex. Stir gently until the Chex are evenly coated with chocolate.
Divide the Chex evenly between two gallon zip bags. Add ¾ cup powdered sugar to each and then shake them until the sugar coats the Chex. Spread it out onto waxed paper to let it cool. Store it in a zip bag or airtight container in the refrigerator.