Combine the first 7 ingredients plus 2 tablespoons cilantro; mix lightly but thoroughly. With wet hands, shape into 1-in. balls. Place meatballs in a 3-qt. slow cooker coated with cooking spray. Pour butter sauce over meatballs.
Cook, covered, on low until meatballs are cooked through, 3-4 hours. Top with remaining cilantro.
Freeze option: Omitting remaining cilantro, freeze cooled meatball mixture in freezer containers. To use, partially thaw in refrigerator overnight. Microwave, covered, on high in a microwave-safe dish until heated through, stirring gently and adding a little water if necessary. To serve, sprinkle with remaining cilantro.
Look for butter chicken sauce in the Indian foods section.