Soak the peas in water overnight, and then drain. Put the peas in a pot with the turmeric and baking soda and add enough water to just cover the peas. Bring the mixture to a boil, and then reduce to a simmer and cook until the peas are soft. Sprinkle with salt, black pepper and red chili powder. Let the peas cool to room temperature.
Mix the peas with the cilantro and potatoes and a generous amount of chaat masala in a bowl. Your filling is ready.
For the pani:
Mix the lemon juice, tamarind, masala, sugar, cumin powder, mint powder, sprinkle of chili powder and salt in a bowl with 4 cups tap water. Pour the pani into a glass jar and refrigerate.
For the puri:
Mix together the sooji, wheat flour and salt in a bowl. Add enough water to make a soft dough. Cover and let it sit for about 30 minutes. Meanwhile, pour 1 inch of oil into a large deep skillet or wok. Heat the oil over medium heat until a deep-fry thermometer registers 350 degrees F. Roll the dough into approximately 1 1/4-inch-diameter discs. Slip the discs into the hot oil one at a time, working in batches, flipping them with a slotted spoon as they puff up until light brown. Drain on a paper-towel-lined-tray and let cool. They will continue to crisp as they cool.
To serve, put 6 to 8 puris on each plate with separate bowls of filling and pani on the side. Provide a small spoon. Lightly tap the top of a puri with the tip of the spoon handle to break and make a small hole wide enough to fill it. Fill the puri halfway with some filling, then spoon the pani into it and pop it into your mouth. Repeat with the remaining puri, filling and pani. Enjoy the delicacy of the pani puri.