Marinate the chicken: Toss chicken with soy sauce, rice vinegar, and hoisin (if using). Let it sit 10–15 minutes while you prep the rest. Yes, you can multitask—trust yourself.
Coat the chicken: Put cornstarch in a bowl. Dredge each piece until it’s lightly crusted and ready for the hot oil party.
Heat the oil: Fill a deep skillet or wok with oil to about 1 inch. Heat to 350°F (180°C). If you don’t own a thermometer, drop in a bit of cornstarch—if it sizzles, you’re good.
Fry the chicken: Fry in batches until golden and crispy, about 3–4 minutes per batch. Drain on paper towels. Don’t crowd the pan; give each piece space to shine.
Make the sauce: In a small bowl, whisk ketchup, brown sugar, stock, sesame oil, garlic, ginger, chili, and extra rice vinegar. Adjust sweetness or heat to taste.
Combine: Pour the sauce into the pan or a separate saucepan, simmer briefly, then toss in the crispy chicken until well coated. If it looks a little glossy, that’s the good stuff.
Finish and garnish: Sprinkle sesame seeds and green onions. Serve immediately—this dish doesn’t like to wait around.