In a small saucepan, combine all ingredients except cornstarch and chicken. Bring to boil over high heat. Reduce heat to low and stir until sugar is dissolved, about 3 minutes. Remove from heat and let cool. Discard cinnamon stick and mix in 1/2 cup water.
Place chicken in a heavy-duty sealable plastic bag. Add soy sauce mixture, seal bag, and turn to coat chicken. Refrigerate for at least an hour, ideally overnight.
Remove chicken and set aside. Pour mixture into a small saucepan. Bring to a boil over high heat, then reduce heat to low. Mix cornstarch with 2 tablespoons water and add to pan. Stir until mixture begins to thicken, and gradually stir in enough water (about 1/2 cup) until sauce is the consistency of heavy cream. Remove from heat and set aside.
Preheat a broiler or grill. Lightly brush chicken pieces on all sides with sauce, and broil or grill about 3 minutes per side. While chicken is cooking, place sauce over high heat and bring to a boil, then reduce heat to a bare simmer, adding water a bit at a time to keep mixture at a pourable consistency. To serve, slice chicken into strips, arrange on plates, and drizzle with sauce.
