Dredge the chicken pieces in flour seasoned with salt and pepper. In a large frying pan, heat the oil and cook the chicken chunks 3 to 4 minutes or until done. Work in batches if necessary to avoid crowding the pan. Drain the chicken on paper towels and set aside, covered to keep warm.
Combine the sugar and soy sauce in a saucepan and heat uintil the sugar dissolves. Add the chicken chunks. Toss the chicken in the sauce with the sesame seeds and scallions. Serve at once.