In a small bowl, whisk together the orange juice, soy sauce, rice vinegar, sugar, cornstarch, and orange zest until smooth and clump-free, then set aside.
Combine the flour, cornstarch, water, egg, salt, pepper, and baking soda in a large bowl until just combined.
Add the chicken pieces to the batter and toss until they are coated evenly.
Heat a deep frying skillet or Dutch oven over medium-high heat and pour in enough oil so that the oil is 3-4 inches deep.
Toss the chicken in the batter, then remove one piece at a time, allowing any excess batter to drip off. Carefully fry the chicken in several batches. Fry for about 5-6 minutes until the chicken is crispy but still pale.
Remove the chicken from the oil and place them on a baking rack with a sheet pan underneath to catch the excess oil. Repeat until all of the chicken has been fried.
Reheat the frying oil to 325 degrees Fahrenheit and fry the chicken a second time, in 2-3 batches, until each piece is golden brown, crispy, and fully cooked. Remove the chicken and set aside on the same baking rack, allowing any excess oil to drip onto the baking sheet.
Heat 2 tablespoons of vegetable oil in a large wok over medium heat. Sauté the garlic and ginger for 30 seconds or until fragrant, then stir in the red pepper flakes and cook for an additional 30 seconds.
Pour the orange sauce into the wok and bring to a simmer, then reduce the heat to medium-low and cook for 3-4 minutes until it begins to thicken and become glossy.
Add the cooked chicken to the pan and toss to coat in the sauce.
Serve over rice and garnish with freshly chopped green onion and sesame seeds, if desired.