Marinate the Chicken: In a bowl, mix the chicken with soy sauce, garlic, and ginger. Let sit for 10 minutes.
Prepare the Coating: In a separate bowl, mix cornstarch and sesame oil. Coat marinated chicken pieces in the mixture.
Fry the Chicken: Heat vegetable oil in a wok or frying pan over medium-high heat. Fry the chicken in batches until golden and crispy (about 4-5 minutes per batch). Remove and drain on kitchen paper.
Prepare the Sauce: In the same pan, stir-fry garlic, ginger, and red chillies for 30 seconds. Add the sauce ingredients (soy sauce, rice vinegar, sugar, ketchup, and chili flakes) and stir until combined.
Combine & Serve: Toss the crispy chicken in the sauce until well coated. Garnish with spring onions and sesame seeds. Serve immediately with steamed rice or noodles.