Line a cookie sheet with parchment paper or a silpat baking mat. Place 1 cup powdered sugar in a large gallon sized ziploc bag.: The faint waxy smell of parchment and the clarity of a clean baking mat set the stage visually, and the tactile act of spreading a single layer of paper helps you judge how much space the final batch needs. Having a dedicated cup of powdered sugar in a large bag means you can immediately move to shaking while the coating is still tacky. A common mistake is skipping the lining which makes cleanup harder and can lead to sticking, so always prepare the sheet ahead of time.
Place milk chocolate chips, peanut butter, and butter in a large bowl. Microwave for 1 minute, stir, then heat in additional 30 second increments, stirring after each, until mixture is melted and smooth. Place Chex cereal into the peanut butter chocolate mixture and stir to coat.: As the milk chocolate chips and peanut butter melt together, the aroma will shift from faintly sweet to deeply chocolatey, and the surface will become glossy. Stirring after each microwave interval disperses heat and prevents scorching. When you fold in the Chex cereal , you should hear soft clicks as pieces separate and see every flake become luxuriously coated. If your microwave overheats the chocolate, it can seize and become grainy, so take your time heating briefly and stirring thoroughly.
Place the cereal mixture into the powdered sugar bag and seal. Shake to coat the cereal. Turn it out onto prepared cookie sheet. Let the muddy buddies harden and cool.: When you seal the bag and shake, you will hear a muffled susurration as cereal pieces rub. The powdered sugar will cling and mattify the glossy coating, transforming the surface to a delicate, pale finish. Pouring the coated pieces onto the lined sheet allows air to circulate, and within minutes the chocolate will firm slightly. Avoid shaking too aggressively or you can crush pieces, and make sure to spread them into a single layer so cooling happens evenly.
Optional: once they cool, give them a toss in another coat of powdered sugar (up to 1 cup) for a super white look.: After the initial set, the surface will feel firm yet still tender under your fingers, and a second dusting deepens the nostalgic, snowy appearance. The extra layer also adds a powdery mouthfeel that some people crave, and visually it makes the batch look bakery fresh. A frequent error is adding the second coat while still warm which creates clumping, so wait until they are fully cooled before re coating.
Once cooled, place muddy buddies, peanuts, raisins, and M&Ms into a large gallon size bag. I added 1 cup of raisins but if you love them, add up to 2 cups. Shake to mix, then serve or seal for later use. Store in the airtight bag/container for up to 5 days.: The final mixing step combines contrasts in texture and temperature, and when you shake the bag you will hear crunches and occasional chewy thumps where the raisins mingle. Packaging in an airtight container keeps the pieces crisp and preserves their aroma. One common slip up is storing while still slightly warm, which introduces moisture and can soften the cereal, so ensure full cooling before sealing for storage.