Chop the pearl onions. Clean the prawns,devein them and wash under running water. Cook the prawns in pressure cooker(or any normal vessel) adding salt, chilly powder and turmeric powder. Remove and chop into bite sized pieces. Heat butter in a pan; add pearl onion and fry till golden brown. Add rice and fry for 2 minutes. Put onion-rice mixture in rice cooker and cook adding water, salt and cardamom powder. When rice is almost done, add the cooked prawns and cook completely and serve hot.