While the potatoes are cooling off, continue with the vada. Add the ginger, garlic and chilli to a pestle and mortar and mash them into a paste. Pick the curry leaves and roughly chop the coriander. When the potatoes are ready, heat some vegetable oil in a small frying pan. Add the mustard seeds and, as soon as they begin to sizzle vigorously, add the curry leaves and asafoetida followed by the ginger, garlic and the chilli paste. Sauté everything for around a minute until the garlic smells aromatic, then stir in the turmeric and turn off the heat. Add the mashed potatoes as well as the lemon juice, coriander and 1/2 tsp salt. Tip it into a mixing bowl and, once cold enough to handle, use your hands to mix it evenly. Divide and shape the mixture into 8 balls, then flatten each ball into a thick potato patty.
Heat the vegetable oil in a sauce pan to around 170 °C (338 °F). In a mixing bowl, whisk together the ingredients for the batter along with 1/3 tsp salt and 100 ml water. Coat 4 of the patties in the batter, adding them to the hot oil right away. Make sure that they don’t overlap or they might stick to each other. Fry the patties until golden and crisp on both sides (around 4 min), turning them every now and then, then remove with a slotted spoon onto a rack lined with kitchen paper and continue with the rest. When you’ve finished the potatoes, drizzle around 4 tbsp of the batter into the hot oil. They will puff up into light crispy bites called chura, which we’ll use as a garnish later.
Only fry the remaining chillis if you've done this before and are confident with deep-frying, as they will spit in the hot oil. Reduce the temperature to around 140 °C (284 °F) and pierce each chilli with a small knife to prevent them from bursting. Then place them on a slotted spoon and carefully dip it in and out of the hot oil, making sure to lift it up as soon as they sizzle. Continue this way until they stop sizzling in the oil and are blistered all over. Transfer to some kitchen paper to drain the excess oil.
Slice the buns in half, spread the coriander chutney over one side of the bun and the tamarind chutney over the other. Then sprinkle the dry garlic chutney over both. Place the vada on top followed by the chura and a fried chilli. Close the bun and serve straightaway with some more of the chutneys on the side.