Place a sieve over a large bowl and line it with muslin cloth. Add the yogurt, wrap the cloth gently, and refrigerate for 3 to 4 hours or overnight until the excess whey drains away and the yogurt becomes thick.
Measure the strained yogurt. You should have about 300 g or 1 1/3 cups hung curd. Set it aside.
Pulse the pistachios in a grinder until coarsely ground. Transfer them to a bowl. Grind the biscuits into fine crumbs and add them to the same bowl.
Add melted butter and cardamom powder to the pistachio-biscuit mixture. Mix until the crumbs look evenly moistened.
Grease the sides of a 6-inch removable base tin with oil. Transfer the crumb mixture to the tin and press it firmly into an even base using the back of a spoon. Chill for 20 to 30 minutes.
Add saffron strands to warm milk and crush them lightly with the back of a spoon to release their colour and aroma. Set aside.
Preheat the oven to 180°C or 350°F. Transfer the strained yogurt to a mixing bowl, then add condensed milk and saffron milk. Whisk gently until smooth and even.
Pour the yogurt filling over the chilled pistachio crust. Shake the tin gently to level the surface.
Wrap the bottom of the tin tightly with aluminium foil and cover the top with another piece of foil. Place the tin inside a larger baking tray and add hot water to create a bain-marie.
Bake for 40 to 45 minutes at 180°C or 350°F. After 40 minutes, remove the top foil and check the center. If it still looks wet, bake uncovered for 5 more minutes.
Switch off the oven and let the baked yogurt rest inside for 15 minutes. Remove it, cool to room temperature, then chill for 5 to 6 hours or overnight.
Demould carefully, slice cold, and garnish with chopped pistachios or dried rose petals before serving.
