In a bowl, mix soy sauce, vinegar, sugar, hoisin, garlic powder, ginger, red pepper flakes, water, and cornstarch. Set aside.
Dip chicken into egg, then coat with cornstarch evenly.
Heat oil in a skillet or wok over medium-high heat. Fry chicken until golden brown and crispy, working in batches if necessary. Drain on paper towels.
Pour out most of the oil, leaving about 1 tablespoon. Add sauce mixture to the pan and cook until it thickens, stirring frequently.
Add fried chicken back to the pan and toss to coat with sauce.
Serve hot, garnished with green onions and sesame seeds if desired.