Marinate prawns with turmeric powder, pepper powder, chilli powder, garam masala powder, lime juice and salt and fry it in oil to a golden brown color.
In the same oil used for frying prawns, add sliced onion and split green chillies and saute it till the onion becomes light brown.
Add ginger-garlic paste and fry for 2 mins.
Add sliced tomato, 1/2 cup chopped mint leaves, 1/2 cup chopped coriander leaves, turmeric powder, garam masala and salt.
Saute till the masala gets mixed well.
Add the fried prawns along with little water and cook the curry in a medium flame until the gravy thickens and remove from fire.
Wash and soak basmati rice for 2 hours and then drain it.
Heat 4 tbsp of ghee in a heavy bottom pan and fry cashew and raisins and remove it from the ghee. Then fry the sliced onion till it is golden brown and crispy and keep it aside.
Cook rice in boiling water along with salt, cardamom, cinnamon and bay leaves. Drain and keep aside
In a large heavy bottomed vessel, spread the prawn masala first and layer the rice over it.
Sprinkle the remaining 2 tsp garam masala over the top of the rice layer, sprinkle Biriyani essence and saffron mixture. Put chopped mint and coriander leaves.
Make one more layer of prawns and rice in the same manner. Put chopped mint and coriander leaves, fried cashews, raisins and spread it over.
Cover the rice with a tight lid and put the pot on very low heat for 10 mins.
Kerala Prawns Dum Biriyani is ready.