In a large mixing bowl, combine the boneless, skinless chicken thighs with the beaten egg, cornstarch, all-purpose flour, salt, and black pepper. Mix until every piece is well-coated. Allow to marinate for 15 to 30 minutes.

Pour vegetable oil into a large skillet, filling it about 1 inch deep. Heat over medium-high heat to 350°F (175°C).

Carefully place the marinated chicken into the hot oil in batches, frying for 5 to 7 minutes until golden brown and cooked through. Transfer to a paper towel-lined plate.

In a separate bowl, whisk together soy sauce, rice vinegar, granulated sugar, hoisin sauce, chicken broth (or water), and cornstarch until well combined.

In another skillet, heat vegetable and sesame oil over medium heat. Add ginger and garlic, sautéing for about 30 seconds until fragrant.

Pour the sauce mixture into the skillet with the ginger and garlic. Bring to a simmer, stirring for 2 to 3 minutes until thickened.

Taste the sauce and stir in red pepper flakes to match your preferred heat level.

Once the sauce has thickened, gently toss the fried chicken in the sauce until fully coated.

Let the chicken and sauce simmer together for 1 to 2 minutes on low heat.

Serve hot, garnished with sesame seeds and chopped green onions.