Best Butter Chicken Recipe Ever (Better Than a Restaurant!)

For marinade:

Combine all ingredients in a zip-top bag or shallow baking dish, massaging the marinade into the chicken.

Let stand at room temperature while you prepare the sauce, or marinate in the fridge overnight.

For sauce:

Heat oil (or ghee) over medium heat in a large saucepan or dutch oven.

Add onions and slowly cook until golden, about 20 minutes, reducing heat if they are getting crispy or browning quickly.

Add garlic and ginger and cook until fragrant, about 1 minute. 
Stir in garam masala, paprika, cumin, cinnamon, cayenne and salt; cook 1 minute more to let the spices toast.
Add tomatoes; cook 2 minutes, then add cream.
Optional: carefully puree using an immersion blender or regular blender.
The sauce base will probably look quite thick at this point (see notes below).

To assemble the curry

Add the chicken to the sauce, along with the marinade.

Cover, and simmer over medium-low heat until chicken is cooked through, about 12 minutes (remove a couple of pieces to make sure they are no longer pink inside).

A gentle simmer is required to gently cook the breasts so they don't become tough, and you don't want to overcook them.

Stir in butter, taste and add more salt if needed. Serve sprinkled with cilantro, if desired.