Toss chicken pieces in cornstarch until evenly coated. Shake off any excess.
Heat oil in a large skillet or wok over medium-high heat. Add chicken in batches and cook until golden brown and crispy, about 3–4 minutes per side. Remove and set aside.
In the same pan, remove excess oil leaving about 1 tablespoon. Sauté garlic and ginger for 30 seconds. Add soy sauce, water, brown sugar, hoisin sauce (if using), and red pepper flakes. Bring to a simmer.
Return cooked chicken to the pan and toss in the sauce. Simmer for 2–3 minutes until the sauce thickens and coats the chicken.
Remove from heat. Garnish with sliced green onions and serve hot over rice or noodles.