It's called thandai (ठंडाई - literally: coolant.)
It's traditionally served with bhang (edible cannabis - which is legal in India!) but even without that it's truly terrific. It's very sweet but you can cut back.
2 cups milk
8 cashew nuts
8 blanched almonds
1/2 tsp fennel seeds
1/2 tsp black pepper
4 long peppers (pippali)
4 green cardamoms
4-6 dried rose buds
1 tsp poppy seeds
pinch of saffron
2 tbsp sugar (or more to taste!)
Note 1: The CC has gone easy on the sugar. It's much more sugary than this.
Note 2: The rose buds are not optional. That's what gives the dish its characteristic fragrance. You can find them in your local Chinatown.
Note 3: It also sometimes features magaz seeds (edible kernel of watermelon seeds) but it's reasonably exotic and the CC left it out. Add 8 of them if you have them.
Note 5: The recipe has connections to Mughal culture. The Mughal Emperors were massive opium addicts. Oddly, alcohol wasn't much in fashion possibly because of Muslim edicts. The same logic of catalysis and sugar applies to opium.
Note 6: All the spices used are considered "cooling spices" in Indian cooking which makes sense since Holi comes on the cusp of the absurdly hot Indian summer.
Note 7: The black pepper and long pepper are "hot spices" and they're there for "balance". A ridiculous concept but you must enter into the medieval Indian mindset.
Soak all the ingredients in the milk for at least two hours. Blend in a blender finely.
Strain and serve cold.