In a large mixing bowl, add boiled potatoes, tamarind pulp, lemon juice, puchka masala, roasted cumin powder, black salt, red chilli powder, and crushed green chillies.
Using a sturdy spoon or your clean hands, mash and mix the ingredients together. Don't over-mash the potatoes. Keeping them slightly chunky gives the chaat a better texture and the mixture should be moist, but not watery. Taste and adjust the black salt or chilli to your liking—it should be bold!
Toss in the boiled kala chana and most of the chopped coriander leaves. Now, take some puchkas and crush them right into the mixture with your hands. Use a light hand to fold the crushed puris and chana into the potato mix.
This is the Churmur! The goal is to incorporate the ingredients while keeping the puri pieces crunchy.
Transfer the chaat to small serving bowls. Garnish immediately with a generous sprinkle of remaining crushed puchkas and fresh coriander leaves.
Serve right away to enjoy the contrast between the tangy, spicy mash and the irresistible crunch of the puri.