Set broiler on high. Adjust oven rack 6 inches from heat. Pat the chicken breasts dry with paper towels. Pound them to an even thinness. Place on a foil-lined baking sheet. In a small bowl, combine the mayonnaise, Dijon mustard mustard, salt and pepper. Spread over the chicken. Broil until bubbling, 3 to 4 minutes. Adjust rack to middle position and broil until cooked through, 5 to 7 minutes.