Heat an oven to 450°F (230°C). Line a broiler pan with foil and set a rack on top of it. In a small bowl, combine butter and ½ teaspoon salt. If desired, add herbs or other flavorings to the butter.
Season both sides of the chicken breasts liberally with salt and pepper. Using a finger, carefully loosen the skin from the meat and slide the butter underneath it. Using the back of a spoon, carefully spread the butter evenly under the skin of each breast.
Rub the skin of each breast with vegetable oil. Place the breasts on the rack and roast until the skin is golden brown and the meat is cooked through, 30-35 minutes. Transfer to a cutting board to rest for 5 minutes.
Carve the breast off the breastbone and rib cage, then slice cross-wise. Serve immediately.