In a saucepan over medium heat, combine soy sauce and sugar. Stir until dissolved, then bring to a boil. Let it simmer for 2-3 minutes until slightly thickened, then add crushed garlic, chopped green onion, sesame oil, chili flakes, and white pepper. Boil for an additional minute before removing from heat.

In a mixing bowl, coat cubed chicken with 4-5 tablespoons of the prepared garlic sauce, cornstarch, and vegetable oil. Ensure chicken is evenly coated. Cover and marinate for at least 30 minutes.

In a separate bowl, whisk together flour, cornstarch, and salt. Gradually mix in reserved garlic sauce until small clumps form, creating a crispy coating.

Heat vegetable oil in a large frying pan over medium-high heat until shimmering. Dredge marinated chicken in the dry coating and fry in batches for 5-7 minutes until golden brown and crispy. Drain on paper towels.

Return the sauce to low heat until warm. Toss the fried chicken in the warm sauce or serve separately.