Soak the potatoes in 2 1/12 c water and 1 tsp salt for 20 minutes. Dry. Fry potatoes on medium heat in butter until browned on both sides. Roast all items in group 1 separately. Grind group 1 with group 2, with a little water, into a fine paste. Brown onions in butter. Add group 3 and paste made from groups 1 & 2. Cook on slow fire Beat in half the yogurt. Add fried potatoes and cook 5 minutes. Add rest of yogurt and 1/2 c water. Cook on slow fire for 20 minutes. Garnish with coriander and chillies, if desired.