In a large mixing bowl, coat the chicken pieces with cornstarch until fully covered.
Heat the vegetable oil in a frying pan or wok over medium-high heat until hot.
Carefully add the coated chicken pieces to the hot oil in batches, frying them until golden brown and cooked through (about 4-5 minutes per batch). Use a slotted spoon to transfer the cooked chicken to a paper towel-lined plate to drain excess oil.
In the same pan, add the minced garlic, ginger, and red chili flakes. Stir-fry for about 1 minute until fragrant.
In a separate saucepan, combine the soy sauce, rice vinegar, sugar, chicken broth, and sesame oil. Bring the mixture to a gentle simmer over medium heat.
Once simmering, add the cornstarch-water mixture to the sauce, stirring until it thickens (about 2-3 minutes).
Add the crispy chicken back to the pan with garlic and ginger. Pour the thickened sauce over the chicken and toss well to combine.
Garnish with sliced green onions and additional chili flakes if desired. Serve hot over steamed rice.