Place the chicken in the slow cooker with salt, sugar, spices, and tomato sauce. Cover with the lid and cook on HIGH for 3 hours or LOW for 6 hours.
Whisk together the cornstarch and heavy cream. After the initial cooking time, pour in the heavy cream mixture and stir to combine. Return the lid to the slow cooker and cook for 30 minutes more on HIGH or 45 minutes more on LOW to thicken the sauce.
After the gravy has thickened, swirl the butter into the, well, butter chicken.
Taste the butter chicken and add more salt if you would like to – at least a pinch or two at the end of the cooking process can brighten up the dish considerably.
Serve butter chicken over rice, with naan for dipping, and with cilantro leaves for topping if you’d like. YUM.