Sauce-Marinade: Combine the sesame oil, vinegar, hoisin, and soy.
Marinate Chicken: Remove 2 tsp of the Sauce-Marinade and combine with the chicken. Stir in the garlic and ginger and let the chicken marinade for half an hour.
Dust chicken: Toss the chicken in the cornflour, making sure to keep the pieces apart so they coat completely.
Shake off excess: Pour the leftover cornflour into a sieve and shake to get rid of it (or take handfuls and shake so the cornflour gets between your fingers).
Finish Sauce: Add cornflour and sugar to the leftover Sauce-Marinade (that you prepared in Step 1). Add the chicken stock and continue to blend.
Heat oil: In a deep skillet (or big saucepan, depending on your comfort level), heat 2 cm (4/5″) of oil to 200°C/390°F.
Fry: Cook chicken for 3 minutes, flipping halfway through, until crispy and golden. Empty into a plate lined with paper towels.
New/clean skillet: Wipe off the oil and put the skillet back on the burner. Alternately, use a different sizable skillet.
Stir Fry Sauce: In a big skillet, heat the oil over medium heat. When the garlic turns light golden, add the ginger and chili flakes and sauté for 30 seconds. Pour in the sauce, bring it to a simmer, and stir it every now and then until it gets thick enough to make a trail across the bottom of the pan.
Toss: Add the chicken and toss briskly to coat; the chicken will stay crisper the faster you work! Move to a serving plate, top with a garnish, and serve right away with your preferred rice.