In a large skillet toast whole spices over medium heat. Stir constantly to avoid burning. Once cumin seeds begin to pop and mixture becomes fragrant, transfer to small bowl to cool. Once cool, grind whole spices in a spice grinder until fine.
In a blender combine ground spice mixture with chili powder, turmeric, 1 tsp of brown sugar, 8 cloves of garlic, ½ ginger, and ¼ cup of vinegar. Pour in ½ cup of water. Blend until smooth. Scrape down sides of blender as needed.
Pour mixture over the chicken pieces and marinate in refrigerator for at least 1 hour, and up to 24 hours.
Heat oil in a large stockpot over medium-high heat. Add onions and saute for 25 minutes, or until very softened and golden brown. Meanwhile, grate the remaining 8 cloves of garlic and remaining ginger using a microplane grater. Mince chiles until fine. Add garlic, ginger, and chiles to onion mixture and saute an additional 5 minutes.
To onion mixture, add potatoes, chicken, and any remaining marinade, and 2 cups of water. Stir until combined.
Bring mixture to a boil. Cover and reduce heat to medium-low. Cook, stirring occasionally, for 25 minutes.
Uncover and boil mixture for an additional 10 minutes, or until sauce is reduced to your liking.
Stir in 1 tsp brown sugar, additional 2 tbsp of vinegar, and salt to taste.
Garnish with sliced chiles.