Whisk together hoisin sauce, orange zest, orange juice, soy sauce, sugar, sesame oil, and 1 tablespoon cornstarch with 2 tablespoons water until smooth; set the sauce aside.
Make the wet batter: in a medium bowl combine 1½ cups of the flour with the ice-cold 1 cup water and the egg; whisk until smooth and the consistency of pancake batter.
Make the dry dredge: in a separate bowl combine the remaining 1 cup of flour (from the 2½ cups total), 1/4 cup cornstarch, 1 teaspoon salt, and 1/2 teaspoon black pepper; mix well.
Heat about 2 inches of vegetable oil in a wok or deep pot to 350°F (175°C).
Working in batches, dip the chicken pieces into the wet batter, then dredge them in the dry flour mixture to coat; set the coated pieces aside on a tray.
Fry the battered chicken in batches until golden brown and very crispy, about 4–6 minutes per batch depending on size; remove with a spider or slotted spoon and drain on paper towels.
Carefully discard the frying oil and wipe the wok clean with paper towels (or transfer chicken to a clean wok); add about 2 tablespoons vegetable oil and heat over high heat.
Add the minced ginger and garlic to the hot wok and stir-fry for about 30 seconds, stirring constantly until fragrant.
Pour in the reserved sauce, stir frequently, bring to a boil, and simmer for 2–3 minutes until slightly thickened.
Add the fried chicken pieces to the wok and toss to fully coat them with the sauce; cook for 1 minute to heat through.
Serve immediately over steamed rice and garnish with chopped scallions if desired.