Mix the soy, hoisin, vinegar, chilli and sesame oil.
Take out 2 tablespoons of Sauce-Marinade and mix with chicken. Add ginger and garlic to chicken, mix, then marinate 30 minutes.
Add cornflour into chicken and toss to coat, ensuring pieces are separated so they get fully coated.
Tip into a colander and shake to remove excess cornflour.
To the remaining Sauce-Marinade, add sugar and cornflour. Mix, then add chicken stock and mix.
Heat 2 cm / ⅘" oil in a deep skillet to 200°C/390°F.
Cook chicken 3 minutes, turning halfway, until golden and crispy. Drain on paper towel lined plate.
Discard oil, wipe skillet and return to heat. Or use another large skillet.
Heat oil in a large skillet over medium heat. Add garlic, ginger, chilli flakes, sauté 30 seconds until garlic is light golden. Add Sauce, bring to simmer and stir occasionally until it thickens enough.
Add chicken, toss to coat quickly and transfer to serving plate, garnish and serve immediately with your rice of choice.