In a large bowl, combine the Rice Chex cereal and broken pretzel pieces.
Melt the peanut butter and chocolate together, either on the stovetop or in the microwave.
Pour the peanut butter–chocolate mixture over the cereal and pretzel mixture and stir to combine.
Chill in the refrigerator until the mixture just barely begins to set, about 30 minutes.
Remove the mixture from the refrigerator.
Add 1 cup (120 g) of the powdered sugar to the chilled cereal mixture; you can mix in a fresh bowl if you prefer.
Gently stir until the powdered sugar is evenly distributed, tossing to coat the cereal.
Chill the mixture again in the refrigerator until set, about another 30–60 minutes.
Add more powdered sugar, ¼ cup (30 g) at a time, until the cereal and pretzel mixture is coated.
Add the M&Ms and peanut butter cups and stir to combine.
Transfer to a serving bowl and serve.