Measure out 7 cups of chex cereal in a LARGE mixing bowl. Set aside.
In medium sized microwave safe mixing bowl, combine the chocolate chips and butter. Microwave on 50% power at 1 minute intervals, stirring until melted.
Add in the peanut butter, microwave again on 50% power at 1 minute intervals, stirring until melted.
Add in the marshmallows, microwave on 50% power at 1 minute intervals, stirring until melted - the marshmallows will not melt completely, but will get puffy. The mixture will begin to get very thick but stir with a spatula.
Scoop this mixture over the chex cereal. Use a spatula (and some elbow grease, it is THICK!) to coat the cereal. You can also use your hands.
Once the cereal is well coated, transfer the cereal to a 9x13 inch baking pan and press it into the pan with your hands.
Allow the bars to cool completely - putting them in the fridge can speed up this process.
Cut the bars into squares. Place 1 cup of powdered sugar in a ziplock bag. Place one bar inside the bag, close and shake until the bar is coated in powdered sugar. Repeat with all the bars.