Marinate the Chicken: Mix chicken, soy sauce, egg, and salt. Let sit 20 mins.
Make the Coating: Combine cornstarch and flour in a bowl.
Coat the Chicken: Dredge marinated chicken in the dry flour mixture.
Heat the Oil: Heat oil in a pan to 350°F (175°C).
Fry in Batches: Fry coated chicken until golden and crisp.
Drain Excess Oil: Place chicken on a wire rack or paper towels.
Mix the Sauce: In a bowl, whisk soy sauce, vinegar, hoisin, sugar, broth, and cornstarch.
Cook the Sauce: Heat a wok. Add garlic, ginger, and pepper flakes. Sauté briefly.
Add Sauce to Wok: Pour sauce into the wok and cook until thickened.
Combine Chicken with Sauce: Add fried chicken to the wok. Toss to coat well.