Place the chex into a large sealable bag so it is ready for the chocolate and peanut butter, set aside.
Place chocolate and peanut butter into a large microwave safe bowl. Microwave for 45 seconds and stir. Microwave for 20 second intervals until the chocolate is smooth. Add the vanilla and stir. Allow the chocolate a minute to slightly cool.
Pour the chocolate over the chex, seal the bag, and gently shake to coat. Be very careful not to stir too vigorously because it will break the cereal. A few clumps of cereal is fine.
Before adding the powdered sugar make sure that the chocolate is not too hot. If it is still pretty warm allow it a minute or two with the bag open to cool. This will help the sugar not immediately be absorbed and will give a better powdered sugar coating.
Add the powdered sugar. Seal the bag and gently shake to coat.
Store in an air-tight container.