Soak urad dal and moong dal in enough water overnight. In the morning rinse the dal, drain the water and set aside.
Transfer the dals to a blender and grind to a smooth paste using little water. [around 1/4 cup]
Transfer batter to a bowl and add 1/2 teaspoon salt to it.
Using a spoon whip the batter for 7-8 minutes till it becomes fluffy and airy. This step is very important in order to make soft bhallas.
Heat oil in a wok/kadai on medium heat.
To make the vada, wet your hands and take a 1-2 tablespoon batter [depending on how big or small you want your vadas to be]. Place 2-3 raisins in the center and close with the batter.
Drop the batter gently into the oil.
Cook till vada is golden brown on each side. Repeat till all batter is finished. Drain on a kitchen towel.
Drop the fried vadas into a pan of lukewarm water for around 20 minutes.
To Serve
Remove vada from water and squeeze it between your hands to remove water. Squeeze gently so that you don't break it.
Take yogurt in bowl and 1/2 tablespoon of sugar to it [optional].
Whisk the yogurt till it's creamy and smooth. Add little water, around 2-3 tablespoons to make it pouring consistency.
Pour the whisked yogurt over the vadas. The vadas should get totally submerged in the yogurt. Pour tamarind-date chutney over the yogurt.
Sprinkle roasted cumin powder, red chili powder, chaat masala and black salt to taste over the yogurt.
Garnish with chopped coriander leaves and pomegranate seeds.
Chill the dahi vadas in the refrigerator until ready to serve.