Fry in oil over medium heat (in a wok or a skillet) until golden brown and crispy. Drain excess oil. Sprinkle cayenne over the chicken and keep warm on low heat. Meanwhile, in a saucepan, combine sauce ingredients (sugar, soy sauce, pineapple juice, vinegar, garlic, and ginger) and heat gently.
When sauce is smooth and sugar has dissolved, SLOWLY add cornstarch mixture and stir constantly until sauce has thickened to desired thickness. Pour sauce in the wok and stir to coat chicken. Stir in green onion and hot red pepper flakes (optional). Remove from heat and serve with fried rice.