Add the mint leaves, coriander leaves, green chili, and a small amount of water into a blender. Blend until you achieve a smooth paste.
In a large bowl, combine the mint-coriander paste, diced onion, diced tomato, cumin powder, chaat masala, black salt, roasted cumin seeds, and lemon juice.
Introduce the chilled water and thoroughly mix the ingredients. Adjust the seasoning and salt according to your preferences. Allow the pani to chill in the refrigerator for a minimum of 30 minutes to allow the flavours to meld.
Prepare the Filling:
In a bowl, meld the boiled and mashed potatoes, cooked chickpeas, finely diced onion, cumin powder, chaat masala, and salt. Ensure a thorough mix to create the filling.
Carefully create a small opening in the top of each puri using your thumb.
Fill each puri with the prepared filling.
Utilize a spoon or a small funnel to pour the chilled pani into each stuffed puri.
Serve the pani puris promptly while they retain their crispness and flavour.