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.