Spray a 9x13-inch baking dish with cooking spray.Or you can line it with parchment paper.
Place chocolate chips, peanut butter, and butter in a large microwave-safe bowl.Microwave at 30 second intervals, stirring after each one, until mostly melted.
Add marshmallows and microwave another 30 seconds. Stir to melt the marshmallows.
Stir in chex cereal and transfer mixture to prepared baking dish, spreading it out evenly.
Let set until firm. Note: You can refrigerate it to get it to set faster.Cut into squares.
Place powdered sugar in a large ziptop bag. Add squares a few at a time and shake until coated.