Combine the soy sauce, sugar, sherry, vinegar, garlic, ginger and red pepper flakes and stir until the sugar dissolves. Transfer to a resealable plastic bag and add the chicken. Seal the bag and marinate the chicken in the refrigerator, turning once, for 1 hour. The chicken can be marinated for up to 4 hours.
Heat the broiler to high. Arrange the chicken on a broiler pan skin side down and broil until brown and crispy, 8 to 10 minutes. Flip the chicken and broil until almost cooked through, about 8 minutes longer. Sprinkle with sesame seeds and cook until the seeds turn golden brown and the chicken is done, 1 to 2 minutes longer.