If you have an immersion blender, add the tomatoes, onion, garlic, ginger, yogurt, garam masala, turmeric, cumin, coriander, chili powder, salt and pepper to your slow cooker and use the immersion blender to puree them until smooth.
If you don’t have an immersion blender, place all of the same ingredients in a regular blender or food processor and puree until smooth. Then transfer the sauce back to your slow cooker.
Add the chicken and stir to coat it in the sauce.
Sprinkle the butter evenly over the chicken.
Cover and cook on low for 6-7 hours or on high for 3-4 hours, until the chicken is tender and cooked through.
(Optional) If you want to thicken the sauce, whisk 2 tablespoons of cornstarch into the heavy cream before adding it to the chicken in Step 7.
About 30 minutes before serving, stir in the heavy cream and season with additional salt and pepper, if needed.
Serve the butter chicken over cooked basmati rice. Garnish with chopped cilantro. Enjoy.