In a large bowl, combine the all-purpose flour, cornstarch, salt, and black pepper. Beat the egg in a separate bowl.

Dip the chicken cubes into the beaten egg, then dredge them in the flour-cornstarch mixture. Make sure each piece is well-coated.

Heat the vegetable oil in a deep fryer or large pot to 350°F (175°C). If you don’t have a thermometer, drop a small piece of chicken into the oil—if it sizzles immediately, the oil is ready.

Fry the chicken pieces in batches, making sure not to overcrowd the pan. Fry for about 4-5 minutes until golden brown and crispy. Use a slotted spoon to remove the chicken and place it on paper towels to drain excess oil.

In a small bowl, whisk together the soy sauce, rice vinegar, sugar, hoisin sauce (optional), sesame oil, and cornstarch. Add 2 tablespoons of water to thin the sauce slightly.

Heat a wok or large skillet over medium heat. Add a teaspoon of sesame oil. Add the minced garlic, ginger, and dried red chilies. Stir-fry for about 30 seconds until fragrant (be careful not to burn the garlic).

Pour the sauce mixture into the wok and bring it to a simmer. Stir constantly as the sauce thickens, about 2-3 minutes. If the sauce gets too thick, add a little water to adjust the consistency.

Once the sauce has thickened, add the fried chicken pieces to the pan. Toss the chicken in the sauce to coat evenly. Stir-fry for an additional 1-2 minutes to heat through.

Transfer the chicken to a serving dish and garnish with sesame seeds or chopped green onions (optional). Serve with steamed rice or vegetables.