Add chicken pieces to the slow cooker, followed by onion, garlic, and ginger.
In a medium bowl, whisk together coconut milk, tomato sauce, garlic powder, curry powder, turmeric, paprika, salt, pepper, and brown sugar. Pour over the chicken and stir gently to coat.
Cover and cook on Low for 6 hours or High for 3 hours, until the chicken is tender and the sauce is rich and fragrant.
In a small bowl, whisk cornstarch and cold water. Stir the mixture into the curry and cook on High for 15 minutes, uncovered, until the sauce thickens slightly.
Spoon the chicken and curry sauce over rice or with naan bread. Garnish with fresh cilantro.