Whisk together the 1/2 cup cornstarch, 1/4 cup flour, 1 tsp baking powder, 1/2 tsp salt, and 1/4 tsp white pepper. Mix in the 1 large egg and 3 tbsp cold water to form a thick batter, then add the chicken pieces and toss to coat.

Heat neutral oil to 350°F (175°C) in a heavy pot to a depth of 2 inches. Fry the chicken in batches of 5 to 10 pieces for 3 to 4 minutes per batch until pale golden, letting the oil return to temperature between batches. Drain on a wire rack.

Raise the oil to 375°F (190°C) and fry all the chicken together for 1 to 2 minutes until deep golden and crisp. Drain on the wire rack.

Mix the 1 tbsp cornstarch and 2 tbsp cold water into a slurry and set aside. Heat the 2 tsp vegetable oil in a skillet over medium heat, add the 3 cloves minced garlic and 1 tsp grated ginger, and stir for 30 seconds. Add the 1/2 cup orange juice, 1 tbsp orange zest, 3 tbsp soy sauce, 2 tbsp rice vinegar, 3 tbsp sugar, 1 tbsp hoisin sauce, and 1/2 tsp red pepper flakes. Bring to a boil, stir in the slurry, and simmer for 1 minute until glossy. Remove from heat and stir in the 2 tsp sesame oil.

Add the fried chicken to the sauce and toss over medium heat for 1 minute until glazed. Plate, top with sliced green onions and sesame seeds, and serve immediately.