Position a rack about 6 inches from the broiler and heat the broiler on high. Line a large rimmed baking sheet with aluminum foil and coat the foil with cooking spray.
In a medium bowl, combine the hoisin, sherry, soy sauce, oyster sauce, garlic, five-spice powder, and red pepper flakes.
Trim and cut the chicken into 1-inch pieces. Add the chicken to the sauce and toss to coat. Thread the chicken onto 6-inch skewers, using 3 to 4 pieces for each skewer and leaving some space between the pieces. Arrange the kebabs in a single layer on the prepared baking sheet and broil until well browned, about 6 minutes. Flip and continue broiling until the chicken is cooked through, about 4 minutes more. If the chicken begins to burn, move the rack down one level.
Transfer the kebabs to a serving platter or dinner plates, sprinkle with the sesame seeds and scallions, and serve.