Drain both lots of lentils using a sieve and discard the soaking water. Tip into a food processor and add the ground cumin, peppercorns, ginger and green chillies. Blitz to a soft paste, adding 1-2 tbsp water as required.
Spoon the lentil mixture into a bowl and beat vigorously for 5-7 mins, or until the mixture becomes light and fluffy. It’s easier to do this in a food mixer fitted with a beater.
Heat the oil in a wok or karahi over a medium heat, ensuring it is no more than two-thirds full. Stir the bicarbonate of soda into the lentil mixture, then use two dessert spoons to gently scoop and shape dumplings about the size of a small walnut. Working in batches, lower the dumplings into the oil using a slotted spoon.
Fry the dumplings for 3-4 mins on each side, turning with a slotted spoon, until golden. Lift out and drain on kitchen paper. You should have 22-24 dumplings when all the batter is used up. Set aside.
Whisk the buttermilk with an equal quantity of cold water in a large shallow baking dish. Coat the dumplings in the buttermilk mixture and leave overnight in the fridge (they will soften as they soak).
The next day, gently press the dumplings to remove any excess buttermilk and arrange on a serving plate.
To serve, whisk the yogurt with the sugar and enough milk to create a consistency that is just thick enough to coat the dumplings. Coat the dumplings with the yogurt and sprinkle with the ground cumin and chilli powder. Drizzle over some coriander relish and date & tamarind sauce, then sprinkle with the coriander leaves.