In a microwave-safe bowl, combine milk chocolate/peanut butter chips, peanut butter, shortening, and vanilla. Heat for 1 minute at 50% power. Stir. Heat for another minute at 50% power. Stir. Continue heating for 30 second intervals (at 50% power) until everything melted and smooth.
Pour Chex in a large bowl. Pour melted chocolate/peanut butter mixture over them. Stir well until all the cereal is coated.
Pour 1 & 1/2 C. of dry brownie mix in a large Ziploc bag. Add half of coated Chex mix. Toss thoroughly, breaking up any chunks.
Spread out on a baking sheet.
Repeat with remaining 1 & 1/2 C. of dry brownie mix and rest of coated Chex mix. Spread out on a baking sheet.
Allow mix to dry. Toss in strainer to remove any excess brownie powder, if necesssary. Serve.