Marinate the prawns in yoghurt, chilli powder and salt for 15 minutes.
Wash and soak the rice for 20 minutes, drain and set aside.
Grind the spice paste ingredients to a fine mixture using a little water (I always dry roast them first for about 2 minutes to bring out their flavours).
Heat oil in a wide and heavy pan, and fry the onions with the dry chillies until golden brown. Add the tomatoes and coriander powder and fry till the oil rises to the surface. Now put in the spice mixture and salt and fry for a further 4-5 minutes.
Tip in the rice and sauté for 2 minutes before pouring in hot water to reach 1 inch above the surface of the rice. Cook the rice for 5 minutes, add the marinated prawns and mix. Now cover and cook on a medium flame for a couple of minutes and then on a slow flame for 10-15 minutes till the rice and prawns are done. Serve with pappadums.