1. Soak 1 1/2 cups of urad dhal (black lentils) with 4 cups of water for 2 hours. 

2. After 2 hours, drain the water completely and discard the water.

3. Use a blender or grinder to make the dhal into smooth paste by adding 1/4 cup of water at a time. By adding all the water at once, the batter can turn coarse.

4. Transfer the batter to a bowl, and add 1 tsp of salt and 3/4 tsp of baking soda. Mix well for 15 seconds so that the salt and soda are mixed well with the batter. Do not overmix as this may release the trapped air and can flatten the vadas. Overmixing can also cause the vadas to absorb more oil.

5. Scoop a spoonful of batter onto wet hands, slightly flatten and make a hole in the center of the batter. Transfer this to the oil carefully. 

6. Fry the vadas until golden brown on each side. The frying process might take around 6 to 7 minutes.

7. Let the vadas cool down completely to room temperature.

8. Take 4 cups of whipped thick yogurt in a mixing bowl. Add 3/4 tsp chili powder, 3/4 tsp cumin powder, 3/4 tsp coriander powder, 3/4 tsp salt, 4 Tbsp sugar. Mix well with a whisk.

9. Once the vadas are to room temperature, put the vadas in the yogurt sauce we just prepared and let it soak for about 3 hours in the refrigerator. Make sure to dunk the vadas in the yogurt.

10. After three hours, Dahi vadas are ready to serve. Vadas become twice as big after soaking for three hours. 

11. Serve by garnishing with shredded carrots, cilantro, and freshly grated coconut. Just before serving, temper 1 tsp of mustard seeds, three red chilies, and 6-8 curry leaves in 1 Tbsp of hot oil. Pour the tempered mixture onto Dahi vadas