Prepare the ingredients by chopping the onion, garlic, ginger, and gathering spices.
In a large bowl, combine chicken thighs, onion, garlic, ginger, garam masala, cumin, paprika, salt, and pepper. Mix well.
Transfer the chicken mixture into the crockpot. Pour in the tomato sauce and coconut milk, and stir until well combined.
Cover the crockpot and cook on low for 6-8 hours or on high for 3-4 hours.
Once cooked, shred the chicken pieces slightly for better flavor distribution. Stir in brown sugar, and adjust seasoning if necessary.
Serve the butter chicken over rice or with naan, and garnish with fresh cilantro.