Soak the potatoes in a large bowl of water for about 1 hour. Prick them over once or twice during soaking so that they are quite soft. Drain and pat dry with kitchen paper (paper towels).
Meanwhile soak the chillies in a bowl of water for 1 hour, then drain and transfer to a small blender. Process, adding a little water if necessary, to make a paste.
Put the asafoetida in a small bowl, add the milk and allow to soak until required.
Heat the ghee in a kadhai or heavy-based pan over high heat, then reduce the heat to low. Add the potatoes and fry for about 15 minutes, or until half cooked. Remove with a slotted spoon and drain on kitchen paper (paper towels).
Pour out all but 2 tablespoons of ghee, then return the pan to the heat, add the cardamom pods, cloves, cinnamon, chilli, and ginger pastes and fry gently for 2-3 minutes.
Remove the pan from the heat and add the yoghurt. Stir well, then return to low heat and simmer for 5 minutes. Add the potatoes, sugar, bay leaves, and turmeric, and season with salt.
Stir, then cover and simmer for about 5-10 minutes, or until the potatoes are cooked.
Add the asafoetida and grated khoya and stir well until the potatoes are evenly coated with the sauce. Remove from the heat and stir in a tablespoon of hot ghee or butter before serving.