In a microwaveable bowl, place the butter and peanut butter and microwave at 20 second intervals until melted. Mix and add in chocolate chips. If chocolate chips don't melt all the way while stirring, place back in microwave for another 20 seconds.
In a very large bowl, pour half of the crispix cereal, pour half of the chocolate mixtures and stir gently with a spatula. Keep stirring until coated in the chocolate. Place on a sheet tray and coat the rest of the cereal in the remaining chocolate chips mixture. Place this on the sheet tray with the other batch. Let cool for at least 20 minutes.
While cereal is cooling, Pick out allllll of the marshmallows from the Lucky Charms. Yes, this is tedious but worth it! Set aside.
It is easiest to use a paper bag for this next step but a plastic baggie is okay too. Place about a cup of powdered sugar in the bottom of paper bag. Add half of the cereal mixture in the bag and close tightly. Turn it upside down and around, over and over until all of the cereal is coated in the powdered sugar. Open the bag and if not all of the cereal is coated, add some more powdered sugar and shake again. Pour half of the marshmallows in the bag and do one last shake. Store in an air tight container. Do this step again with the remaining mix.