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.
Notes
Grind separately: Grind the spices and nuts in two different batches to prevent the nuts and spices from over-grinding and releasing their oils.
Make-ahead: The Thandai masala can be prepared 2-3 weeks in advance. Store it in an airtight container in a cool, dark place or freeze for up to 2 months. Thaw in the fridge overnight, if frozen.
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 are following any diet plan, I recommend cross-checking with your preferred nutrition calculator.