Rinse the dals with water and soak overnight in 3 to 4 cups water.
In the morning, drain the water and transfer the drained dal to a blender along with 1/3 cup water (approx), salt and cumin seeds. Grind the mixture to a smooth paste.
Then transfer the batter to a stand mixer fitted with whisk attachment. You can also use a hand mixer here or use your hands and mix with a whisk/spoon.
Beat at medium speed for 5 minutes (using the whisk attachment) until the batter doubles in size and becomes fluffy. It will also become lighter in color. If mixing by hands, whisk for around 10 minutes.
To check if the batter is ready for frying, drop some of it in a jar of water, it should float which means it's airy and ready to fry.
Heat oil in a kadai on medium heat. Oil should neither be too hot, nor too cold. If it's too hot- bhalla will become brown quickly but will be raw from inside.
Once the oil is heated, use a ice cream scoop or use your hands to drop batter into the hot oil. Do not overcrowd the kadai, give enough space for the bhallas to fry.
Fry the bhalla on medium heat, stir often. Fry until they turn golden brown, this will take around 8-9 minutes on medium heat. Remove fried bhalla on paper towel. Repeat until all batter is finished.
Let the bhallas cool. Once cooled, add fried bhalla to warm water for 20 minutes. Bhalla will swell up and increase in size.
After 20 minutes, take each bhalla and squeeze gently between your palms to drain water, repeat with all the bhallas.
Meanwhile whisk 2 to 3 cups plain yogurt. Add little water or milk to thin it down. Add bhalla to a bowl and then cover with the whisked yogurt. Let it sit in the refrigerator for at least 2 hours.
Whisk the remaining 3 cups yogurt and add some salt and sugar to it (to taste). The yogurt should be thick but pouring consistency.
Take out the bhalla from the refrigerator, place each in serving tray. Then pour the whisked yogurt all over. In total I used around 5 to 6 cups of yogurt here.
Sprinkle cumin powder, red chili powder and salt on top.
Top the dahi bhalla with cilantro chutney, sweet chutney, sev and serve!