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1
Drain the soaked urad dal thoroughly and grind to a thick, smooth batter using the minimum water necessary — the batter must hold its shape when dropped from a spoon, so that the vadas fry firm and do not splay into the oil. Transfer to a bowl and beat vigorously by hand or with a whisk for 4–5 minutes, working air into the batter so the finished vadas are light inside rather than dense. The batter is sufficiently aerated when a small portion dropped into a cup of water floats to the surface rather than sinking.
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2
Heat the oil in a deep vessel to 170–180 °C; below this temperature the vadas absorb oil rather than sealing quickly on the outside. Wet your hand, take a portion of batter, and slide it gently into the oil to form a rough disc approximately 5 cm in diameter, frying in batches without crowding so the oil temperature does not drop and the vadas colour evenly. Turn the vadas once and fry until they are deep golden on all sides and fully cooked through — a vada at this stage sounds hollow when tapped lightly against the side of the vessel.
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3
Remove the fried vadas and transfer immediately to a vessel of room-temperature water, not cold water, which tightens the exterior and prevents the interior from softening properly; submerge and leave for 10–15 minutes so the soak drives out the frying oil and converts the dense fried crumb to a pillowy, yielding texture. This is where the dish lives or dies: vadas that skip this step remain oily and chewy no matter how well the rest is executed. At the end of soaking, lift each vada and press very gently between the palms to expel excess water without compressing the interior.
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4
Whisk the cold yoghurt with the sugar, kala namak, and salt until smooth, keeping it cold so it stays thick enough to coat the back of a spoon and envelop each vada rather than pooling beneath it. Taste the yoghurt at this point: it should be distinctly sweet with a clean, faintly sulphurous background note from the kala namak, which tells you the seasoning is right before the dish is assembled. If the yoghurt is loose, it will slide off the vadas and the dish will look flat and watery on the plate.
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5
Arrange the squeezed vadas in a single layer in a cold serving dish and spoon the sweetened yoghurt over them generously, covering each vada completely so no fried surface is exposed to air and the vadas continue to soften gently underneath. Drizzle the tamarind chutney and green chutney over the surface in alternating lines, then dust with roasted cumin powder and red chilli powder. The finished dish should present distinct bands of colour — white yoghurt, dark tamarind, bright green chutney, and ochre spice — with no element obscuring another.