Place the chicken pieces, diced onion, minced garlic, and grated ginger into the slow cooker.
Add the tomato sauce, sliced butter, garam masala, cumin, turmeric, paprika, and salt. Stir to combine everything evenly.
Cover and cook on low for 6 to 7 hours until the chicken is tender.
Stir in the heavy cream during the last 30 minutes of cooking.
Serve the butter chicken over the cooked basmati rice and sprinkle with chopped cilantro.