1. Preheat broiler to high.
2. Place chiles on a foil-lined baking sheet; broil 10 minutes or until blackened, turning once. Wrap in foil; let stand 10 minutes. Peel chiles; cut in half lengthwise. Discard seeds and membranes. Cut chiles into strips; place in a bowl. Add salt; toss.
3. Sprinkle half of cheeses evenly over 4 bread slices; top with chiles, remaining cheese, and remaining bread slices.
4. Heat a large cast-iron skillet over medium heat. Coat both sides of 2 sandwiches with cooking spray; place in pan. Cook 4 minutes or until golden brown and crisp. Turn over; cover and cook 3 minutes or until cheese melts. Remove from pan. Repeat procedure with remaining sandwiches and cooking spray.