Tie the curd overnight in a muslin cloth. In the morning you will see that all water has gone and what is left is just thick curds. Put this is a flat bowl, add sugar, saffron and start mixing and beating it for about 1/2 hour till sugar is dissolved and its become thick. After 1/2 an hour keep it in the fridge for some time till it becomes more thick.
Keep stirring for another 15 mins, add elaichi powder (optional since maybe saffron essence may go off) and chopped pistas.
Transfer in a glass bowl and refrigerate till you serve. Since the curd is sour, you will get a tangy taste also.