In a bowl or large ziplock bag, combine chicken pieces with all of the MARINADE ingredients, cover with plastic wrap, and let marinate for 30 minutes or overnight in the fridge.
Place a large skillet over medium-high heat. Add two tablespoons of olive oil.
When hot, add chicken pieces in batches, ensuring not to overcrowd the pan. Cook until browned, for about 3-5 minutes per side. Set aside on a plate and cover with foil to keep it warm.
Melt the butter. Add the onion, garlic, and ginger, stir, and cook for 2 minutes.
Add the aromatics: cumin, garam masala, red chili powder, sugar, and paprika, and cook for 1 minute.
Transfer the mixture to the slow cooker. Add the chicken.
Add the tomato sauce and evaporated milk. Season with salt and pepper, and stir to combine.
Cook low for 6 to 7 hours or on high for 3-4 hours.
Thirty minutes nearly the end of the cooking time, open the lid and stir in the heavy cream.
Serve with rice garnished with cilantro.
