Sauce-Marinade: Combine the soy sauce, hoisin sauce, rice vinegar, chilli paste, and sesame oil in a bowl.
Marinate Chicken: Reserve 2 tablespoons of the Sauce-Marinade and mix with the chicken pieces. Add the finely grated ginger and garlic to the chicken, stir well, and let it marinate for 30 minutes.
Dust Chicken: Coat the marinated chicken pieces with cornflour, ensuring each piece is thoroughly covered. Separate the pieces to ensure they are fully coated.
Shake off Excess: Place the coated chicken in a colander and shake off any excess cornflour. Alternatively, you can grab handfuls of the chicken and shake off the excess cornflour through your fingers.
Finish Sauce: Add the brown sugar and cornflour to the remaining Sauce-Marinade, mix well, then add the chicken stock and stir until combined.
Heat Oil: Heat about 2 cm (4/5") of oil in a deep skillet or large pot to 200°C/390°F. Refer to Note 4 for tips on using less oil.
Fry: Fry the chicken pieces for about 3 minutes, turning them halfway through, until they are golden and crispy. Drain the fried chicken on a plate lined with paper towels.
New/Clean Skillet: Discard the used oil, wipe the skillet clean, and return it to the heat. Alternatively, use a new large skillet.
Stir Fry Sauce: Heat the oil in a large skillet over medium heat. Sauté the finely chopped garlic, ginger, and red chilli flakes for about 30 seconds until the garlic turns light golden.
Toss: Add the prepared sauce to the skillet, bring it to a simmer, and cook until it thickens enough to draw a path across the pan base. Quickly add the fried chicken to the sauce, toss to coat thoroughly, and immediately transfer to a serving plate. Garnish with your choice of finely sliced green onions or sesame seeds and serve with your preferred rice.