Cut the chicken thighs into uniform 1-inch pieces, then dredge in batches through cornstarch, shaking off any excess and setting aside on a plate.
In a separate bowl, whisk together the water, soy sauce, rice vinegar, hoisin sauce, sugar, and 1 1/2 tablespoons cornstarch to create the sauce base.
Heat oil in a non-stick skillet over medium heat. Fry the chicken in 2 to 3 batches until each piece is golden and thoroughly cooked. Add more oil between batches if necessary. Transfer cooked chicken to a bowl. Leave approximately 1 tablespoon of oil in the skillet.
Introduce garlic, ginger, and red pepper flakes to the hot oil. Sauté briefly until aromatic, about 30 seconds.
Pour in the sauce mixture and bring to a gentle simmer, stirring continuously. Let it cook for a minute to mellow the vinegar’s sharpness. Taste and adjust with extra soy sauce or vinegar if preferred.
Return the chicken to the pan, tossing to coat thoroughly in the thickened sauce.
Serve immediately, garnished with sesame seeds if desired.