In a large microwave safe bowl melt chocolate chips, peanut butter, and butter for 30 seconds and then stir. Continue microwaving in 15 second increments, stirring after each time, until chocolate is completely melted. Stir in vanilla.
Gently fold in chex cereal being careful not to break up the pieces until it's evenly coated.
Add powdered sugar to a large zip top bag and then add cereal mixture. Seal the bag and shake well until each piece is evenly covered with powdered sugar.
Transfer chow back to a clean bowl and fold in M&Ms.
Store in an airtight container.
The calories shown are based on the recipe being divided 15 ways, with 1 serving being about 3/4 of a cup. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.