Wash and soak 1/2 cup udad dal and 2 tbsp. mung dal in water for atleast 4 hours. Then drain and grind with very little water to make a smooth paste. Take the paste in a big mixing bowl and add salt, little ginger paste and black pepper pwd. Whisk well for 5 minutes.
At this point if you drop a blob of the paste in a bowl of water, it should immediately float up. This means the batter is light and airy and well whisked.
Heat oil to deep fry the vadas. And keep a big bowl of thin buttermilk ready. Add a little salt and red chilli pwd to the buttermilk. Drop spoonful of batter in the oil and fry on medium heat till golden brown. Drain and immediately put the vadas in the buttermilk. Fry all the vadas similarly in small batches.
To serve - whisk cold yogurt (I like to use Fage) with salt and sugar till smooth. Put the vadas in the serving bowl. Top generously with the yogurt. Sprinkle some red chilli pwd and cumin pwd. Add a dash of green mint chutney and dates-tamarind chutney. Finally garnish with chopped cilantro. You can also add little shev on top. Serve immediately.