Pat the chicken pieces dry with paper towels. In a bowl, toss them with about a tablespoon of soy sauce so the coating sticks a little better.

Place the cornstarch in a large zip-top bag or shallow dish. Dredge the chicken pieces in the starch until they're lightly coated on all sides. Shake off any excess.

Heat the oil in a deep skillet or Dutch oven to 350°F (175°C). Fry the chicken in batches until golden brown and cooked through, about 3–4 minutes per batch. Transfer to a rack or plate lined with paper towels to drain.

In a separate pan, combine the remaining soy sauce, hoisin, sugar, rice vinegar, garlic, ginger, and red pepper flakes. Cook over medium heat, stirring, until the sauce thickens slightly, about 2–3 minutes.

Add the fried chicken to the sauce and toss to coat evenly. If the sauce seems a little thick, splash in a teaspoon or two of water to reach a glossy, clingy coating.

Finish with a drizzle of sesame oil, if using. Garnish with sliced scallions and sesame seeds. Serve immediately with steamed rice or vegetables on the side.