Add the chicken thighs to the slow cooker.
In a bowl, stir together the tomato sauce, garam masala, cumin, paprika, and salt until blended.
Pour the sauce over the chicken and add the butter on top.
Cover and cook on low for 6 to 7 hours until the chicken is very tender.
Shred the chicken lightly with two forks right in the slow cooker, then stir in the heavy cream.
Warm the naan bread in the oven at 350 degrees for 3 to 5 minutes.
Serve the butter chicken with the warm naan and sprinkle cilantro on top.