In a medium microwave-safe bowl, combine the chocolate chips and peanut butter and heat in the microwave on high in 30-second intervals, stirring well between each interval, until melted and smooth.

In a large mixing bowl, combine the chocolate mixture with the Rice Chex and toss gently with a spatula until the cereal is coated. Pour the brownie mix over top of the cereal and toss until coated.

Add the M&M's, peanut butter cups, and pretzel twists and toss gently to combine.