Spray an 8-inch pan with nonstick cooking spray and set aside. Melt butter in large saucepan over low heat. Add marshmallows and stir until melted, then remove from heat. Add potato chips and stir until well coated. Spray a spatula or your hands with nonstick cooking spray and press the mixture evenly into the prepared pan. Sprinkle candy bar pieces on top of chips and press down gently to adhere. Sprinkle with flaky salt to finish. Refrigerate until chocolate sets, 15 to 20 minutes. Cut into 2-inch squares.