Cut chicken into 1-inch cubes.

In a medium bowl, mix together the flour, cornstarch, salt, and pepper. Add the egg, oil, and water, whisking until smooth.

Add the chicken cubes to the mixture, ensuring they are thoroughly coated. Cover and chill for 30 minutes in the fridge.

In a medium saucepan or deep fryer, add 2 inches of oil and heat to 350˚F (177˚C). Line a plate with paper towels and set aside.

Add the marinated chicken cubes in batches of about 10-15 pieces, shaking off excess batter if needed. Deep fry for 2-3 minutes until golden, turning halfway through with a slotted spoon for even browning.

Remove the chicken to the plate and cover with foil to keep warm while making the sauce.

To a medium saucepan, add orange juice, orange zest, soy sauce, vinegar, brown sugar, and garlic. Place over medium heat and bring to a low boil.

Carefully taste a few drops and adjust sugar, soy sauce, and vinegar to suit your preference.

Mix the cornstarch and water in a cup to make a slurry and pour into the sauce. Stir constantly until the sauce coats the back of a spoon.

Add the fried chicken and toss until the sauce coats it thoroughly.

Garnish with optional sesame seeds, green onions, and red pepper flakes. Serve immediately.