In a blender, combine the mint leaves, coriander leaves, green chili, and a little water to make a smooth paste.
In a large bowl, mix the mint-coriander paste, chopped onion, chopped tomato, cumin powder, chaat masala, black salt, roasted cumin seeds, and lemon juice.
Add the chilled water and mix well. Adjust the spices and salt to your taste. Refrigerate the pani for at least 30 minutes to allow the flavours to meld.
Prepare the Filling:
In a bowl, combine the boiled and mashed potatoes, cooked chickpeas, finely chopped onion, cumin powder, chaat masala, and salt. Mix well to make the filling.
Gently make a small hole in the top of each puri using your thumb.
Stuff each puri with the prepared filling.
Pour the chilled pani into each stuffed puri using a spoon or a small funnel.
Serve the pani puris immediately while they are crisp and flavourful.
