Add the rice to a large bowl and cover with water. Using your hands, mix the rice around to release the starch (watching the water turn a cloudy color).
Drain the rice until the water runs clear. Cover the rice with water one final time, cover and refrigerate overnight.
Finely grind the saffron using a mortar and pestle. In a microwave-safe bowl heat ¼ cup of water until very warm, but not boiling (about 1 minute). Add in the saffron and let sit until the saffron has bloomed and liquid is cooled, at least 15 minutes.
Once the right has been soaking overnight, roughly crumble the grains of rice with your hands until broken apart.
In a large nonstick saucepan, combine the crumbled rice with 9 cups of water and the salt. Bring to a boil.
Reduce the heat to a gentle simmer and simmer over medium-low/low heat, stirring frequently, until the rice is softened, about 30 minutes.
Stir in the saffron-infused water, sugar, cinnamon, butter, and cardamom. Cover and simmer over low heat, stirring occasionally, until the rice breaks down and the pudding is the texture of loose porridge, about 20 - 30 minutes more. Remove the cardamom pods and discard.
Stir in the rosewater. Cover tightly with plastic wrap and chill in the refrigerator until thickened, about 1 hour. Divide until bowls, top with slivered almonds, and serve.
