FOR WADA:
Wash urad dal 3-4 times till water runs clear. Soak it for at least 4 hours in enough water.
Grind the dal with chilies, fried curry leaves, ginger, and salt. Grind it to a smooth batter with little water for easier movement of mixer blade. Keep aside.
Heat oil in a hollow thick bottomed pan/kadhai.
Keep aside a container full of clean water with little salt added to it.
Test drop a small portion of batter in the hot oil. If the batter floats immediately, the oil is at perfect temperature for frying the wadas.
Lower flame and drop tablespoon of batter in the hot oil. Drop enough balls but avoid overcrowding the pan. Increase heat to medium.
Pour hot oil over the wadas with the slotted spatula. Wait till the bottom side if wadas turn golden in color and then flip them. Fry this side till appropriate golden brown color has reached.
Transfer the wadas to a plate covered with paper napkins to drain excess oil. Then transfer them to the container containing salted water.
Similarly fry the rest of the batter and follow the above steps of draining and soaking the wadas.
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FOR FLAVORED YOGURT:
Whip the yogurt till it's smooth.
Blend chilies, ginger, cumin seeds, fennel seeds, fried curry leaves, sugar, and salt till it forms a paste.
Add the paste to the yogurt and mix well.
Add enough water to make thick buttermilk. Refrigerate the buttermilk till it's chilled.
Squeeze excess water from the wadas gently and place them in the chilled buttermilk.
Prepare the tempering. Add oil to hot skillet. Add cumin seeds, curry leaves, and red chili powder. Switch off the gas.
Swirl the tempering in the pan and quickly add it to the dahi wadas. Sprinkle little red chili powder, chaat masala, and chopped coriander leaves over the dahi wadas. Serve immediately.