In a bowl, melt together the butter, peanut butter, and chocolate in the microwave, stirring every 30 seconds until melted and smooth.

Combine the Chex and pretzels in a large bowl. Pour the chocolate peanut butter mix overtop and stir until evenly coated. Pour the cereal and pretzels into a large Ziploc bag. Add the powdered sugar. Seal the bag and shake until the cereal is well-coated. Let dry, then add the M&Ms.