Whisk the yogurt ingredients and set aside. If you feel the yogurt is very thick then add ¼ cup more water.
Blitz the green chutney ingredients in a mini blender along with 2-3 tbsp of water. Adjust seasoning if needed.
Mix your dry masala (if making) and set aside.
Heat the oil in a small pan for the chickpeas on medium heat
When the oil is hot add the mustard seeds and nigella seeds.
Add the chili flakes, give it a quick stir and add the chickpeas
Now add the salt, chili powder, cumin powder and ½ cup of water and cook on low heat till the chickpeas are tender. If they start to dry up then add more water - you want the mixture to have some liquid.
When the chickpeas are tender mix in the chaat masala and set aside.
Assembly: You can start with yogurt first or just layer chickpeas then potatoes then yoghurt. Follow these with your chutneys, sliced onions, papri, chaat masala and/or extra dry masala, cilantro and green chilies. Serve with extra chutneys and chaat masala on the side
Yoghurt: Our family likes a runnier chaat with a yoghurt base, but if you want yours to be drier then reduce the yoghurt by half and drizzle over the chickpeas.
Tamarind: for 3 tbsp pulp heat a small piece (slightly smaller than golf ball) of tamarind in the microwave with ⅓ cup water. Strain. A little more or less liquid is fine. If using Tamarind concentrate use 1 tbsp. You can also use a Tamarind Chutney but I recommend reducing it to 2 tbsp because a.) they are frequently sweetened and b.) the tamarind in it is already cooked down.
Dry Masala: This mix adds a fun layer of flavour. However, if you don't have dry roasted cumin and coriander handy it is absolutely fine to skip it.
Tamarind Chutney: there is a recipe for a tangy version here https://www.flourandspiceblog.com/imli-ki-chutney-spicy-tangy-sweet-no-dates-or-gurh/ but feel free to use storebought