In a large bowl, combine the soy sauce, brown sugar, bourbon, ketchup, garlic, ginger, onion powder, and black pepper. Mix well until the sugar is dissolved.
Place the chicken pieces in the slow cooker and pour the sauce mixture over the top. Stir to coat the chicken evenly.
Cover and cook on low for 4 to 6 hours or on high for 2 to 3 hours, until the chicken is cooked through and tender.
In a small bowl, mix the cornstarch and water to create a slurry. About 30 minutes before serving, stir the slurry into the slow cooker to thicken the sauce.
Once the sauce has thickened, serve the bourbon chicken over cooked rice and garnish with sliced green onions.