Tip the yoghurt into a clean muslin cloth and tie it up tightly with a piece of string right above the top of the yogurt so that the water presses away.
Tie the muslin somewhere where it can hang with a large bowl underneath to catch the water and leave it for 24-36 hours. Try to choose somewhere cool. I like to hang mine in a cool corner of the kitchen. Keep tipping away the water as the bowl fills up.
Remove the yoghurt from the muslin and place it in a large bowl. Add the icing sugar and mix well.
Press the yoghurt and sugar mixture through a fine-holed sieve to remove any lumps.
Mix in the ground cardamom, saffron, mace (or nutmeg) and pistachios if using. Chill in the fridge for at least 8 hours before serving.
This Creamy Kesar Shrikhand keeps well in the fridge for up to a week after it's made.