Cut the chicken thighs into 1-inch pieces, then coat them in cornstarch in batches and gently shake away the extra. The pieces should look evenly dusted rather than heavily caked, with no loose piles of starch on the platter.

Whisk the cold water, low sodium soy sauce, rice vinegar, hoisin sauce, sugar, and cornstarch in a bowl until smooth. The mixture should look thin and cloudy at this stage, with the sugar and cornstarch fully dispersed.

Warm the oil in a non-stick skillet, then fry the chicken in 2 to 3 uncrowded batches until golden on both sides and cooked through; transfer each batch to a bowl. The chicken should sizzle steadily on contact and develop a deep golden crust instead of pale, steamed patches.

Add more oil if needed while frying, leaving at least 1 Tbsp oil in the skillet after the last batch. A light coating of hot oil should remain across the pan to carry the aromatics without letting them stick.

Stir in the ginger, garlic, and red pepper flakes and cook for about 30 seconds. The sharp garlic scent should soften into a warm ginger aroma before the garlic begins to brown.

Pour in the whisked sauce and stir as it comes to a simmer, then let it bubble for 1 minute and adjust with more soy sauce or rice vinegar to taste if desired. The sauce changes from cloudy to glossy and translucent as it thickens enough to lightly coat a spoon.

Return the fried chicken to the skillet and toss it quickly in the hot glaze, then garnish with sesame seeds if desired. Each piece should look lacquered with sauce while its crisp ridges still remain visible.