Rinse the rice 2-3 times with water and soak it in water for half an hour.
Take mango cubes into a blender, blend them to a smooth puree, and keep the mango puree aside.
Take a heavy bottom kadhai or pot and add milk to the kadhai. Bring the milk to a boil over medium-high heat. Stir frequently and keep scraping the bottom and sides of the kadhai to avoid scorching at the bottom.
Once the milk comes to a boil, reduce the flame to low. Remove ¾ cup of milk, and soak a generous pinch of saffron in a ¼ cup of milk. Keep the reserved milk aside.
Simmer the milk on medium heat for 10 minutes. Meanwhile, strain the soaked rice and transfer it to a grinder. Grind the rice to a coarse paste.
6. Add the coarsely ground rice to the milk and cook the rice until the phirni is cooked and becomes thick in consistency. (It will take around 40-45 minutes.)
Stit frequently, and keep scraping the kadhai's bottom and sides regularly until it reaches the desired thickness. (Note: The ready phirni has a thick consistency and is thicker than the kheer.)
Once the phirni thickness, add sugar, saffron milk, cardamom powder and reserved milk. Mix well, cook the phirni for 5 minutes, and turn off the flame. You can adjust the amount of sugar (increase or decrease) as per your taste.
Allow the phirni to cool down at room temperature. Once cooled, add mango puree to it and mix well.
Transfer mango phirni to earthen or regular bowls and cover with cling wrap. Refrigerate for 3-4 hours.
Garnish with slivered pistachios and mango cubes and serve chilled.