Step 1: Cut chicken into 1-inch pieces and pat dry. In a bowl, whisk together 1 tbsp soy sauce, 1 tbsp rice vinegar, 1 tsp sesame oil, and egg white. Add chicken and marinate for 15-30 minutes.
Step 2: In a shallow dish, combine 1 cup cornstarch, flour, salt, and pepper. Dredge chicken pieces, shaking off excess. Place on a wire rack.
Step 3: Heat 2-3 inches of vegetable oil in a deep skillet to 350-375°F (175-190°C).
Step 4: Fry chicken in batches for 4-6 minutes, or until golden brown and cooked through. Remove to a wire rack to drain. Maintain oil temperature between batches.
Step 5: For the sauce, whisk together chicken broth, 1/4 cup soy sauce, 1/4 cup rice vinegar, sugar, hoisin sauce, dark soy sauce (if using), and 1 tsp sesame oil in a bowl. In a separate small bowl, whisk 2 tbsp cornstarch with 3 tbsp cold water to make a slurry.
Step 6: In a clean wok or large skillet, heat 1 tbsp vegetable oil over medium-high heat. Add minced ginger and garlic; sauté for 30 seconds until fragrant. Add red pepper flakes and stir for 15 seconds.
Step 7: Pour the prepared sauce mixture into the wok. Bring to a simmer, then slowly whisk in the cornstarch slurry. Cook, stirring, until the sauce thickens to a glossy consistency.
Step 8: Add the fried chicken pieces to the sauce. Toss gently to coat evenly. Cook for 1-2 minutes until chicken is warmed through.
Step 9: Serve immediately over cooked white rice, garnished with sesame seeds and sliced green onions.