Pour the candy melts into a glass microwavable safe dish. Heat in 30 second increments, stirring occasionally until melted. Let cool a few minutes.
Pour the melted white chocolate over the Chex cereal and gently stir until all the cereal is coated with melted chocolate.
Divide the 2 cups of powdered sugar between the 2 large freezer bags.
Divide the chocolate coated cereal between the 2 bags of powdered sugar, seal shut and gently shake each bag to coat the cereal with the powdered sugar.
Add the M&M's to the puppy chow.
Store the puppy chow in an airtight container in the fridge.