Preheat the oven to 375 degrees F (190 degrees C).

Place beaten egg in a shallow bowl. Place cornstarch and garlic salt into a large resealable plastic bag.

Dip chicken in beaten egg, allowing excess to drip off. Drop chicken into cornstarch mixture, seal the bag, and shake until well coated.

Heat oil in a large skillet over medium heat. Cook chicken in hot oil, turning occasionally, until browned, about 5 minutes. Transfer chicken to a baking dish.

Stir together vinegar, sugar, broth, ketchup, soy sauce, and salt in a medium bowl. Pour over chicken in the baking dish.

Bake in the preheated oven until chicken is no longer pink in the center and the juices run clear, about 40 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).