When butter is heated, fry the curry paste until aromatic and oil splits.
Add spices and stir in (1 or 2 mins)
Add lamb and salt for taste.
Keep stirring, about 2-3 mins to coat lamb over the curry paste.
Pour enough water and continue simmering until lamb is half cooked.
Add potatoes, stir and cook until lamb and potatoes becomes soft and tender.
Off heat and drop in yogurt and coriander leaves.
Gently stir to dilute yogurt.
The curry is done.