Start with the sauce. Add the sugar, vinegar, ketchup, soy sauce, sesame oil, brown sugar, and garlic to a medium saucepan. Whisk everything together and bring it to a boil over medium-high heat.

In a small bowl, mix the cold water and cornstarch until smooth. Pour this into the boiling sauce, lower the heat, and let it gently simmer for about 5 minutes, whisking now and then, until it thickens.

While the sauce does its thing, put the chicken pieces in a large zip-top bag. Add the cornstarch, salt, and pepper. Seal it and shake until the chicken is evenly coated.

Heat the olive oil in a large skillet over medium-high heat. Add the chicken and cook for about 5 to 7 minutes, stirring and flipping so it browns evenly and cooks through. Add a little more oil if the pan looks dry.

Once the chicken is cooked, lower the heat and pour the sauce over it. Stir until every piece is coated and let it simmer together for a minute or two.

Finish with green onions and sesame seeds if you like, and serve right away.