Lentil dumplings in yogurt dahi bhalla

Name: Lentil dumplings in yogurt (dahi bhalla)
Ingredients: 65g skinned white urad dal (dhuli urad), soaked overnight; 65g yellow mung dal, soaked overnight; ¾ tsp ground cumin; ¼ tsp coarsely ground black peppercorns; 40g ginger, peeled and roughly chopped; 2 green chillies, deseeded and roughly chopped; ½ tsp bicarbonate of soda; vegetable or sunflower oil for deep-frying; 1 x 268ml carton buttermilk; 300ml Greek or natural yogurt; 1 tsp caster sugar; 3-4 tbsp milk; 1 tsp ground cumin; ¼ tsp Kashmiri chilli powder; 2 tbsp coriander relish; 4 tbsp date & tamarind sauce; a few coriander leaves