1) Tip the yogurt 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 can drip away.
2) Tie the muslin somewhere where it can hang suspended, with a large bowl underneath to catch the water, and leave it for 24-48 hours. Keep tipping away the liquid as the bowl fills up.
3) Remove the yogurt from the muslin and place it in a large bowl. It should be very thick and crumbly. Add the sugar and mix well.
4) Press the yogurt and sugar mixture through a fine-holed sieve to remove any lumps.
5) Mix in the cardamom powder, saffron, mace and pistachios. Chill in the fridge until you're ready to serve as it is or stuff into the doughnuts.
2) Sift the self-raising flour and bread flour into a large bowl. Add the sugar, butter, milk, sour cream and lemon zest. Use your hands to bring the mixture together into a dough. At first it will appear sticky but continue kneading for around 2 minutes. Stop kneading for 1 minute to allow the dough to relax, add the saffron and then knead for a further 2-3 minutes until the dough becomes smooth and soft. You could also use a dough blade for this if you have one.
3) Grease a bowl twice the size of the dough with any flavourless oil and place the dough inside. Cover with a damp tea towel and allow to rise in a warm place for around 1 hour, or until the dough has doubled in size.
4) Once the dough has risen, knock it back with your fists to remove all of the air inside. If you're making small doughnuts, divide into 30g portions and roll into 28 balls. If you're making larger doughnuts, roll into 14 balls, about 60g per piece.
5) Place onto a greased tray, leaving room around each piece as it will rise again. Cover with a damp tea towel and allow to rest in a warm place for a further 50-60 minutes.
6) Heat enough oil in a pan to deep fry the doughnuts. Fry small doughnuts at 155C until golden, and larger doughnuts at 140C until golden. Fry a few at a time, trying not to overcrowd the pan. Keep agitating the doughnuts to ensure they are evenly coloured all over.
7) Drain doughnuts and coat in a mixture on sugar and crushed pistachios. Allow to cool slightly.
8) To serve, cut a doughnut widthways and pipe with the sweet yogurt mixture. Alternatively, pipe the yogurt directly into the centre of the doughnuts as you would with regular jam doughnuts. Pipe with the yogurt mixture immediately before serving.