In a food processor or blender, combine the diced tomatoes (undrained), quartered yellow onion, minced garlic, minced ginger, garam masala, sugar, chili powder, ground turmeric, ground cumin, and ground coriander. Blend until the mixture is as smooth as possible.
Pour the blended mixture into a 6-quart slow cooker. Add the 1 (8 ounce) can tomato sauce, 2 pounds chicken pieces, and ¼ cup unsalted butter. Stir everything gently to combine.
Cover the slow cooker and cook on high for 3-4 hours or on low for 6-8 hours, or until the chicken is tender and cooked through.
Once the chicken is cooked, stir in ½ cup heavy cream. Continue to cook on low for an additional 30 minutes to allow the sauce to thicken slightly. Season with salt and black pepper to taste.
Serve the creamy slow cooker butter chicken hot over cooked white rice and with warm naan bread. Garnish with fresh chopped cilantro, if desired.