Add half the butter and half the oil in a large nonstick pot over medium-high heat.
Add chicken, cinnamon stick and saute for a few minutes until light golden brown and no longer pink. It does not have to be fully cooked through.
Transfer this to large crockpot (6 quart or larger).
In the same nonstick pot, heat remaining butter and oil over medium-high heat.
Add onion, tomatoes, garlic, ginger, salt, pepper, red chili powder, paprika, coriander powder, cumin powder, curry powder, garam masala, green chili.
Mix until combined.
Saute for a few minutes until all spices are fragrant and onions are partially softened.
Transfer this mixture to crockpot too.
Note: I highly recommend using a multifunctional slow cooker because you can easily to do all these initial stovetop instructions in the crockpot without making another pot or pan or even your stovetop dirty.
Add broth, tomato paste, coconut milk, sugar.
Mix until combined.
Cover and cook on High heat for 3 hours or until chicken is cooked through.
Remove lid and mix everything together.
Optional step if you prefer a very thick curry: In a small mixing bowl, make a cornstarch slurry by dissolving cornstarch powder with some of the liquid and juices from the crockpot. Pour this slurry in the crockpot. Mix until combined. Cook uncovered on High heat for 15-20 minutes.
Garnish with fresh cilantro and enjoy!