Place chicken (fresh or frozen) in a slow cooker. In a small bowl, whisk together brown sugar, apple cider vinegar, garlic, soy sauce and black pepper; pour over the chicken. Cover and cook on LOW for 6-8 hours.
Meanwhile, about 20-30 minutes before serving, cook rice according to package instructions.
Remove chicken from the slow cooker to a large bowl; cover and set aside. Pour remaining sauce into a saucepan and bring to a boil over medium-high heat.
In a small bowl whisk together cornstarch and cold water. Add cornstarch mixture to the sauce. Whisk until the sauce begins to thicken. Remove from the heat and pour over chicken and rice. Sprinkle with red pepper flakes if desired.