In your slow cooker, stir together sugar, soy sauce, vinegar, ginger, garlic and pepper. Place chicken in slow cooker and coat well in soy mixture. Cook on low for 3-4 hours*, turning chicken occasionally.
Once cooked, carefully remove chicken from slow cooker, chop into bite-sized chunks; set aside.
Pour liquid into a medium saucepan and bring to a boil. In a small bowl, whisk cornstarch and water until combined. Slowly add cornstarch mixture to boiling liquid and reduce to a simmer. Allow sauce to thicken for 2-3 minutes then add chicken to the saucepan. Coat chicken with sauce and let heat through.
Serve over cooked rice. Garnish with scallions and/or sesame seeds, if desired.
I used 1 1/2 lbs. of chicken, which was fully cooked in 3 hours.