In a glass bowl, microwave the chips and butter until they are melted and smooth. Stir to combine. Then stir in the vanilla. Pour the melted mixture over the Chex and gently toss until covered as much as possible. Divide the Chex between two large zip bags. Add ¾ cup powdered sugar to each. Shake like crazy until the Chex are coated with powdered sugar. Spread the covered Chex onto a large sheet of waxed paper and let set.
Place in a covered container with mint M&M's. Keep them covered until ready to eat!