In a large, microwave-safe bowl, combine the milk chocolate wafers, creamy peanut butter, and salted butter.
Microwave on high in 30-second intervals, stirring between each, until the mixture is completely melted and smooth. This should take 60 to 90 seconds total.
Pour the Rice Chex cereal into the bowl with the melted chocolate mixture. Gently stir with a spatula until all the cereal is evenly coated.
Add 1½ cups of the powdered sugar to a gallon-sized zip-top bag. Add the coated cereal, then pour the remaining 1½ cups of powdered sugar on top.
Seal the bag, removing excess air. Shake the bag well for about a minute, until all the cereal pieces are thoroughly coated in powdered sugar.
Spread the powdered sugar-coated cereal in a single layer on a large baking sheet lined with parchment paper.
Allow the cereal to cool and the chocolate to set completely, about 30 minutes at room temperature.
Once cooled, transfer the set cereal to a very large mixing bowl. Add the mini pretzels, candy-coated chocolates, and mini marshmallows. Gently toss to combine.
Serve immediately or store in an airtight container.