Cut chicken into 1-inch cubes and toss with cornstarch, tapping off the excess. Set aside.
In a bowl, whisk together all the sauce ingredients.
Heat oil in a non-stick skillet. Cook chicken in batches until golden and cooked through. Transfer to a bowl.
In the same pan, sauté garlic, ginger, and pepper flakes for 30 seconds until fragrant.
Pour in the sauce and simmer for 1 minute. Adjust with more soy sauce or vinegar if needed.
Add the chicken back to the pan, toss to coat, and garnish with sesame seeds if desired.