Pat the chicken pieces completely dry and season with black pepper. Whisk all glaze ingredients except the cornstarch and cold water in a measuring jug and set aside. In a small bowl, mix the cornstarch and cold water into a slurry but do not add it yet.
Heat a 12-inch heavy skillet over medium-high for 2 minutes, then add the oil until it shimmers. Sear the chicken in two batches, undisturbed for 3 minutes, then turn and cook 2 to 3 minutes more. Transfer to a clean plate and repeat with the second batch.
Reduce heat to medium. Spoon out excess fat, leaving about 1 teaspoon in the skillet. Pour in the re-whisked glaze and scrape up the browned bits. Simmer gently for 3 minutes — do not let it boil hard.
Re-whisk the cornstarch slurry and stir it into the simmering sauce for 30 to 60 seconds, until a spoon leaves a brief path that closes after about one second. Remove from heat immediately if it thickens to a paste.
Return the chicken and any collected juices to the skillet. Toss for 2 to 3 minutes over medium heat until every piece is coated and the thickest pieces reach 165°F (74°C) at the center.
Rest the skillet off heat for 3 minutes. Scatter sesame seeds and scallions over the top and serve.