Add peppercorns, cardamom, fennel seeds, poppy seeds, melon seeds, saffron, and rose petals to a spice grinder and pulse to a fine powder. Transfer to a bowl.
Now add cashews, almonds, and pistachios to the spice grinder and pulse to a fine powder. Combine with the spice powder. Thandai masala is ready.
Video
Notes
Keep the nuts cold: Starting with cold nuts prevents them from releasing their oils while grinding.
Grind separately: Grind the spices and nuts in two batches to prevent over-grinding and oil release.
Clumpy powder fix: Sift it through a medium-mesh sieve.
Powder too spicy?: Grind in another teaspoon or two of melon seeds or cashews, then mix into the powder.
Make-ahead: The Thandai masala can be prepared in advance. It can be refrigerated for 3 weeks or frozen for up to 2 months, so pick whichever timeline works best for you.
Double or triple the recipe: Split the thandai ingredients and grind them in batches to prevent the oils from releasing.
Note: The nutrition facts below are my estimates. If you follow any diet plan, I recommend cross-checking with your preferred nutrition calculator.