Thanks for the reply Robin, I know you're into vegetarian stuff and these should be interesting.
I should report on the "real" pani puris that we had last night -- but first I want to mention that I think we enjoyed the ersatz puris with mashed potatoes and salsa even more than the original, and I think there is a huge potential in that direction, filling these crunchy little puffs with western food. There are things that should go together in one bite, that would be delicious in a crunchy little packet. I mentioned foie gras and, what, chopped HB egg and a dab of mayo? Black beans, sour cream, salsa.
At any rate, I had no idea what Chaat Masala was when I wrote the OP here. It turns out it is a blend of spices with salt and pepper, along the lines of Garam Masala. When you open the foil packet it smells like every Indian restaurant, a delicious savory smell. The filling for the puris that I used was a mix of semi-mashed potatoes plus chickpeas, with chaat masala, other spices, and a little minced red onion and minced jalapeño. It tasted familiar, I suppose I was reminded of Aloo Gobi.
I have never had anything like the Pani, it is intriguing but it wasn't instantly addicting. The Pani is the "water" sauce for the puris. It is clear and watery but the flavor is bright, fresh, and intense. You wouldn't want to drink a glass of it, it's too strong tasting, and a teaspoon of that on a pani puri punches up the spicy flavors in the potato filling dramatically. The filling and the Pani are both chilled well before serving, and the overall effect was -- this is something that is designed for a dry throat on a very hot day.
The surprising richness of the flavor comes from the construction -- you start by putting a lot of cilantro and mint leaves into a blender with chopped ginger and jalapeño. Put in a little water, blend to a paste, and then put the paste through a strainer. The initial extract was so hot I thought "Oh No, nobody will be able to eat these!" but there is quite a bit of dilution after that first step and you don't so much notice the hotness, instead it comes off as a brightness that goes with the fresh green flavor of the leaves.
This is closest to the recipe that I used.
https://www.youtube.com/watch?v=FAJ2xCJOIoY