Season the chicken with salt, pepper, and garlic powder.
Place the chicken into the slow cooker.
In a bowl, whisk together the brown sugar, minced garlic, soy sauce, chicken broth, butter, Dijon mustard, and apple cider vinegar.
Pour the sauce over the chicken.
Cover and cook on LOW for 5 to 6 hours or HIGH for 2½ to 3½ hours.
Remove the chicken and set it aside.
Whisk together the cornstarch and cold water until smooth.
Stir the slurry into the sauce.
Cook on HIGH for 10 to 15 minutes until the sauce thickens.
Return the chicken to the slow cooker and spoon the glaze over the top.
Garnish with fresh parsley before serving.