Spray the inside of your slow cooker with non-stick cooking spray. Place chicken inside slow cooker. In a medium bowl, whisk together brown sugar, vinegar, soda, garlic, soy sauce, and pepper, until dissolved. Pour over chicken. Cook on LOW for 6-8 hours or HIGH for 4 hours.
Take chicken pieces out of slow cooker and pour remaining sauce into saucepan. Place saucepan over high heat. Mix together corn starch and water, pour into saucepan, and mix well. Let sauce come to a boil and boil for 2-3 minutes, or until it starts to thicken and turns into a glaze. Remove from heat and let sit for a minute or two (it will thicken as it cools down).
Serve chicken over rice or noodles and top with glaze. Sprinkle red pepper flakes or scallions on top if desired.