Soak the rice in water and let it soak for about 1 hour. Drain and mash the grains with a pestle or something similar. Add the 2 1/2 liters of water and bring to the boil, let simmer gently for about 1 hour, stirring frequently.
Finely grind the saffron threads with a little sugar in a mortar, then dissolve with 2 tablespoons of hot water. Add the saffron, sugar, cardamom and rose water to the rice. Melt the butter and stir in as well.
Put the whole thing in a large bowl, garnish with sliced almonds or pistachios and let cool.