Marinate julienne chicken with salt, black pepper, garlic powder, chilli flakes, egg, cornflour, and plain flour for 15-20 minutes.
Whisk together brown sugar, soy sauce, oyster sauce, chilli sauce, green chilli sauce, chicken broth, and 1 tsp cornstarch in a small bowl for the sauce.
Fry marinated chicken in hot oil in batches until golden and crispy (3-4 minutes). Set aside.
In a wok over high heat, add 2 tbsp oil and sauté ginger garlic paste until fragrant.
Add half of the sliced green chillies and half of the julienne ginger, toss for 30 seconds.
Add fried chicken and pour over prepared sauce mixture. Stir-fry rapidly on high heat until the sauce coats the chicken completely.
Toss in remaining green chillies and julienne ginger. Mix well, top with toasted sesame seeds, and serve hot with rice.