In a bowl, combine the chicken, garlic, soy sauce, and pepper. Mix well and let marinate for 10 minutes.
Add the cornstarch to a large plastic bag, then add the chicken. Shake until all pieces are evenly coated.
Heat oil in a skillet over medium high heat. Fry the chicken until golden brown and fully cooked. Remove and set aside.
In a clean skillet, heat 1 teaspoon of oil over medium heat. Add the garlic and chili flakes and cook until fragrant and lightly toasted, about 2 minutes.
Stir in the soy sauce, hoisin sauce, rice vinegar, brown sugar, and ½ cup of the water. Bring to a simmer.
In a small bowl, whisk together the remaining ¼ cup water and cornstarch. Pour into the sauce and cook for 1 minute, stirring, until thickened.
Add the cooked chicken and toss until fully coated in the sauce. Serve over rice with sesame seeds and green onions.
