In a small bowl combine curry powder, turmeric, coriander, cayenne and salt. Set aside.
Place coconut milk and water in a 4-6 quart slow cooker.
Add the bowl butter, garlic, and the bowl of seasonings. Gently whisk until combined.
Add sweet potatoes, onion and chicken breasts. Stir to combine.
Cover slow cooker and cook on low for 6-8 hours, or on high for 4-6 hours. Stirring every 2 hours.
During the last 30 minutes, remove chicken from the slow cooker and shred it with a fork then place back in the slow cooker.
At the same time, combine cornstarch and water until smooth; stir into slow cooker mixture.
Serve curry over brown rice with a cilantro garnish.