Mix garlic, ginger, turmeric, garam masala, coriander, and cumin in a small bowl. In a large bowl, combine yogurt, salt, and about half of the spice mixture. Whisk well and add chicken, turning to coat. Cover and chill for 4 to 12 hours. Cover remaining spice mixture and chill until ready to use.

Heat ghee or oil in a large pot over medium heat. Cook onion, tomato paste, cardamom, and red pepper flakes, if using, stirring often, until the onions are soft, about 5 minutes. Add remaining spice mixture and cook, stirring often, until bottom of the pot begins to brown, about 4 minutes.

Add tomatoes and their juices, crushing them a wooden spoon as you stir and scraping up browned bits from the bottom of the pan. Bring mixture to a boil, reduce heat, and simmer, stirring often, until sauce thickens, 8 to 10 minutes.

Stir in cream and chopped cilantro. Working carefully, puree mixture with an immersion blender until smooth. Simmer, stirring occasionally, until sauce thickens, 30-40 minutes.

While the sauce simmers, preheat broiler. Line a rimmed baking sheet with foil and set a wire rack on top. In a single layer, place chicken on the rack. Broil until the meat begins to char and blacken, about 10 minutes (it will not be cooked through).

Chop chicken into bite-size pieces and add it to the sauce. Simmer until the chicken is cooked through, about 8 to 10 minutes. Serve with basmati rice and garnish with cilantro and cardamom seeds, if desired.