Add the milk in a heavy-bottomed pan. Keep the heat medium and slowly bring it to a boil. Keep stirring frequently to avoid any burning.
Once the milk comes to a boil, add the Thandai powder to the milk and mix well.
Reduce the heat to low and let the milk cook until it reduces to almost one-fourth of the original quantity and becomes thick. Keep stirring every few minutes so that the milk does not stick or burn. Also, while the milk boils, keep scraping the sides of the pan to remove the milk solids and add them back to the pan.
Do also note that the rabdi will thicken once it cools down as well.
Once the milk has reduced to a desired consistency, add the sugar and rose water and cook for a couple of minutes more.
Remove the pan from heat and let the rabdi cool for about half an hour.
Once cool, refrigerate the rabdi for around 4 hours or until chilled well.
Garnish with slivered nuts of your choice and serve chilled with Shahi Tukda.