The batter will bubble once mixed.
Mix everything except the chicken meat in a large bowl. The batter will be thin. After a minute, it will foam up slightly and be light. Toss the chicken in the batter. Cover it with cling film and put in the refrigerator to chill for at least 10 minutes (and up to 1 hour).
Heat the oil until almost smoking (or up to the highest setting on your deep fryer). Drop the pieces of battered chicken into the hot oil one at a time. Do this very gently. Do not crowd the deep fryer (or pan) or the chicken will not crisp up. You may have to fry the chicken in batches.
Preheat the oven to its lowest setting (175 F or gas mark 1/2). Remove the chicken from the oil when cooked through and dark golden brown (about 5-7 minutes). Drain off the excess oil. Spread the pieces on a pan covered in paper towels. Place the chicken in the oven to keep warm while you finish frying the chicken and make the sauce.
Whisk the cornstarch, soy sauce, sesame oil, rice vinegar, water, ginger, garlic, sriracha, and water together in a bowl. Set aside.
Heat (on high heat) the sugar and 2 tablespoons of water in a large, deep skillet. Watch it carefully. It will bubble. After 7-10 minutes, it will start to caramelize and turn brown. When the sugar starts to turn golden, quickly pour in the other whisked sauce ingredients and continue to whisk until the sauce is smooth. Clumps of sugar may stick to the whisk, but these will melt off after a few minutes. Continue cooking and whisking until the sauce is smooth, translucent and thick.
Carefully add the cooked chicken to the mixture. Add the green onions and sesame seeds. Turn carefully to coat with the sauce. Serve hot!
This is best served immediately because the chicken pieces only remain crisp for a little while once they are drenched in the sauce. However, it still tastes great when served as part of a buffet line. And of course, it makes for great left overs!