Butter chicken curry (Murg Makhani)

Yes, our fantastic meal came from a mix that Hanalee sent to us directly from India. But this is a recipe I found online that seems like it would reproduce it accurately.

Optional marinade for 1 lb chicken pieces overnight
1 Tbsp vinegar or lemon juice
1 tsp coriander
1 tsp cumin
1 tsp chili powder
1 onion chopped very finely, almost paste-like
2 tsp fresh ginger or 1 tsp ginger powder
1 clove crushed garlic
2 drops red food coloring and 1 drop of yellow

For the Sauce
4 large tomatoes, chopped
4 Tbsp butter
1 Tbsp cream
1 tsp coriander
1 tsp cumin
1 tsp chili powder
2 tsp fresh ginger or 1 tsp powder
salt, pepper, and sugar to taste

Heat 2 Tbsp butter on medium level in a heavy-bottomed pan and add the chicken, 1 chopped onion, 3 crushed garlic cloves, and the marinade (optional). Cover and cook for about 25 minutes or till the chicken is fully cooked. After the liquids have evaporated, stir fry the chicken for some time in the residual fat.
Heat 2 Tbsp butter in a second saucepan and add the chili powder, coriander, cumin and black pepper. Fry for a few seconds. Add the chopped tomatoes, sugar, salt and cook uncovered on medium level for about 7 minutes till the puree thickens and the fat separates. Stir in the beaten cream and reduce the heat to low.
Add the chicken and chopped ginger to the simmering sauce. Sprinkle salt to taste if needed. Mix well. Cover and simmer on low heat for about 10 minutes or till the curry is thoroughly heated through.
Serve over hot rice or alone with naan (Indian bread) and the following topping - not optional.

Thinly slice 2 large onions as if making onion peels. Add 2 cups sour cream (or 2 cups fromage frais), 1 Tbsp lemon juice, and lots of freshly chopped cilantro.