Dice chicken into even bite-size pieces and lightly season them with a little salt and pepper.
Whisk the egg in one bowl. Place the cornstarch in another bowl.
Dip each piece of lightly seasoned chicken in the egg, then coat it in the cornstarch. Press gently so it sticks.
Heat the oil in a large pan over medium-high heat until it's hot, about 175°C (350°F) if you have a thermometer. No thermometer? Drop in one piece of coated chicken. It should sizzle right away and start to float within a few seconds.
Fry the chicken in batches. Don't crowd the pan, or the oil cools down, and the coating turns soft instead of crispy.
Cook until golden, about 5 to 6 minutes per batch. It's done at 74°C (165°F) inside, or just cut the biggest piece in half. It should be white all the way through, not pink.
Transfer to a wire rack to drain, not a paper towel lined plate. A rack lets air move underneath the chicken, so the bottom stays crispy instead of steaming and going soft.
Combine the honey, soy sauce, rice vinegar, ketchup, brown sugar, sesame oil, and garlic in a small saucepan. Stir over medium heat and bring to a gentle simmer.
Mix the cornstarch and cold water in a small bowl until smooth. Pour it into the simmering sauce, stirring the whole time. It should thicken in about 1 to 2 minutes, until it coats the back of a spoon.
Add the crispy chicken to the pan. Toss until every piece is coated in the glossy sauce.
Sprinkle with sesame seeds and green onion. Serve right away over Spicy Egg Fried Rice or plain noodles.