Heat milk in a heavy-bottom non-stick pan over medium heat and bring it to a boil. Stir frequently to avoid scorching at the bottom of the pan.
Once the milk comes to a boil, add Thandai powder to the pan and mix well.
Reduce the heat to low and let the milk cook until it is reduced to almost ¼th and is thick and grainy. Stir after every 2-3 minutes to avoid scorching.
Once the milk is reduced, add sugar and rose water to the pan and cook for another minute.
Remove the pan from the heat and let the rabdi cool for 10-15 minutes. Refrigerate the rabdi for 3-4 hours or until chilled. Transfer it to serving bowls, garnish with slivered nuts and serve chilled.