Cut the chicken into bite-sized 1-inch cubes.
Mix together flour cornstarch salt and pepper in a bowl.Add egg vegetable oil and water whisk well until the batter is smooth.
Coat the chicken pieces thoroughly in the batter then cover and chill in the fridge for 30 minutes or at least 10 if pressed for time.
Heat about 2 inches of vegetable oil in a deep pan or fryer to 350°F (177°C) and prepare a plate with paper towels.
Fry the battered chicken in batches to prevent overcrowding for 2-3 minutes turning halfway until pieces are golden and cooked through.Transfer to the paper towels and keep warm.
For the sauce combine orange juice orange zest soy sauce vinegar brown sugar and garlic in a saucepan.Bring it to a gentle boil over medium heat tasting and adjusting the balance if needed.
Slowly stir in the cornstarch slurry while stirring until the sauce thickens enough to coat the back of a spoon.
Toss the fried chicken in the thickened orange sauce coating each piece evenly.
Serve immediately garnished with sesame seeds chopped green onions and red pepper flakes if desired for extra flavor and presentation.