In a pan on med-low heat, combine soy sauce, sesame oil, vinegar, sugar, salt and pepper. Place trimmed scallions in the mixture and cook for about 2-3 minutes, or until simmering. Remove from heat and set aside to slightly cool.
Pound chicken until thin and dry well with paper towel. Season with salt and pepper on both sides.
Place a few scallions on one end of each chicken, and roll up the cutlets. Secure with toothpicks and slice in half
Combine cornstarch and water with the cooled remaining sauce in the pan.
Heat a grill or grill pan to medium. Grill chicken rolls on all sides, while brushing on the sauce, until fully cooked.
Serve with more sauce on top and a general sprinkling of sesame seeds.