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.
Saute the Chicken:
Place a large skillet over medium-high heat. Add 2 tablespoons of olive oil.
When hot, add chicken pieces in batches, making sure 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.
Add the butter, and melt it. Add onion, garlic and ginger, stir and cook for 2 minutes.
Add the aromatics: cumin, garam masala, red chili powder, sugar and paprika, 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:
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.