Set broiler on high.
Adjust oven rack 6 inches from heat. Pound the chicken breasts to an even thinness. Place on a foil-lined baking sheet.
In a small bowl, combine the mayonnaise, mustard, and a pinch each of 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.