In a bowl, combine the cornstarch, salt, and black pepper. Toss the chicken pieces in the mixture until well coated.
Heat the vegetable oil in a large skillet or wok over medium-high heat. Once hot, add the chicken in batches, frying until golden brown and cooked through, about 4-5 minutes per batch. Remove the chicken and set aside.
In the same skillet, add the minced garlic and ginger. Sauté for about 30 seconds until fragrant.
In a separate bowl, whisk together the soy sauce, chicken broth, sugar, rice vinegar, hoisin sauce, sesame oil, and red pepper flakes. Pour this sauce into the skillet and bring to a simmer.
Add the fried chicken back into the skillet, tossing to coat the chicken in the sauce. Cook for an additional 2-3 minutes until everything is heated through and the sauce has thickened slightly.
Remove from heat and garnish with sliced green onions and sesame seeds before serving.