When ready to cook, turn you oven to broil and set your oven rack 6" away from the broiler. Pull the skin back over the chicken thighs. Place the thighs on a rack on a foil covered baking sheet.
Broil for 5-6 minutes a side. The chicken should be browned on both sides, but not cooked through. Set aside. They will finish off in the butter gravy sauce.
In a blender or food processor, add the garlic, ginger, onion, and tomatoes. Puree. Set aside.
In a large sauté pan over medium high heat, melt the butter. Stir in the paprika, garam masala, cloves, cayenne, and cardamom. Cook for 1 minute.
Add in the puree from the blender. Let simmer over low heat for 30 minutes.
Stir in the heavy cream, salt and sugar.
Add the chicken thighs back in. Simmer over medium heat for 15 minutes, or until chicken thighs are cooked through.