Soak almonds, cashewnuts, pistachios, watermelon seeds, fennel seeds, poppy seeds, black peppercorns in a bowl of water, for 4 to 5 hours.
After 5 hours, drain the water and grind all the ingredients to a fine paste. This is the thandai masala. Keep aside.
In a bowl, take 2 tbsp warm milk.
Add a big pinch of saffron to it and keep it aside for an hour.
Heat the milk in a thick bottomed pan.
When the milk starts boiling, lower the heat and keep stirring.
Add thandai masala and keep stirring.
Do this until the milk has reduced to one third of its quantity.
Add sugar, saffron mixture and mix well.
Keep scraping the sides of the pan and mix.
Add cardamom powder and cook for another 3 to 4 minutes.
Let it cool down. Refrigerate thandai rabdi for 30 minutes to an hour.
To assemble the trifle, pour a layer of thandai rabdi in a small glass.
Crumble motichoor laddoos and spread it over the rabdi.
Add another layer of rabdi and top it with a layer of motichoor.
Garnish with almond slivers, pistachio slivers and dried rose petals.
Refrigerate and serve chilled.