Rinse and soak 1/4 cup Basmati rice in water for at least 15 minutes.
Once it has soaked well, discard the water, transfer the rice to the mixer jar. Pulse it to make a coarse paste. If required, you can add a few drops of water. But remember, the texture of the rice has to be coarse and grainy.
Heat 1liter full-fat milk in a big thick bottom pot. Let the milk come to a good boil.
In a small bowl, take a big pinch (little more than a tsp) of saffron strands, add 3 tbsp of hot milk and let it sit for some time.
Add some hot milk to the ground rice, mix well and then add this mixture to the pot with milk, and mix well quickly. This helps to avoid the formation of rice lumps. Keep the heat on low at this stage. In case small lumps are still formed, mash them with a back of a spoon. Add in the saffron milk and mix it well. On low-medium heat, let the milk and rice mixture boil well such that the cream layer is collected on top. Mix in the cream back to the milk and let it boil for a good 20 minutes. The mixture will start thickening as the rice cooks.
At this stage mix in a few tablespoons of water to the prepared Thandai powder, make a thin paste and transfer this to the pot of milk making sure it is on low heat.
Immediately, also add in 3/4 cup sugar and mix everything well. You can do it with the help of a whisk. Let the phirni be on low heat for 5 more minutes. It gets much thicker now with the addition of the Thandai powder. Lastly, mix in 2 tsp of rose water. You can add more if you had not used dried rose petals in the powder.
Take the pot of phirni off the heat and do not cook further. It should be of thick but pouring consistency. Remember, it will turn much thicker on cooling.
If you have small earthen pots, transfer the phirni into those and let them set in the refrigerator for an hour. Serve it chilled.