Pressure cook the yam cubes till they are just tender. 2-3 whistles I would reckon. Keep aside.
Marinate the prawns with a pinch of turmeric powder and salt. Keep aside.
If you are using potatoes, heat 1 tbsp oil in a kadai, fry the potato cubes till they are light golden. Keep aside on a kitchen absorbent towel.
In the same oil, lightly saute the prawns. Keep aside.
Add the remaining oil to the kadai. When smoking hot, add the crushed cinnamon, cardamom, cloves and bay leaves. Allow the spices to releases their aroma.
Now add the chopped onions and saute till they take a tinge of golden brown. Add the chopped garlic, ginger paste, tomato puree, green chillies, sprinkle a little salt. Cook over a medium flame till the tomato is cooked.
Dissolve the turmeric powder, coriander powder, chili powder and cumin powder in just a little water to make a smooth paste.
Add the paste to the kadai, continue to cook over a medium flame till oil starts to separate from the masala.
Add the fried potatoes and pressure cooked yam cubes. Saute for 4-5 odd minutes. Add 1/2 a cup of warm water, sprinkle in the sugar and give it a hearty stir.
Add the prawns and bring to a simmer. Another 5-6 minutes, I would say. Till the potatoes and yam are tender.
Adjust seasonings and finish with a generous dollop of ghee and a hearty sprinkle of garam masala powder.
Serve hot with rice