Make the vada:
Drain the soaked dal thoroughly. Place in a food processor with the ginger and chillies and salt and process into a coarse, fairly solid paste.
Form the vadas by picking up a lump of the batter, about one inch in diameter, and pressing it out on the palm of your hand (I like dampening my palms with some water to prevent the batter from sticking.)
Drop the vadas one by one into the hot oil without overcrowding them. Flip the vada when the underside turns golden-brown. Remove to a rack or a paper towel when the vada is golden-brown all over.
Tip: Always ensure your oil is not too hot or too cold. Too cold oil will cause the food to absorb the oil, which is bad for your waistline, of course, but will also end up in soggy vadas. Oil that is too hot will cause the outside of the vada to brown rapidly, before the inside gets cooked.
For the sauce:
Make the dahi or spiced yogurt:
Place all the ingredients in a blender and process until smooth.
To serve the vadas, pool some of the sauce in the platter, place the vadas in the center, and pour more sauce on top. I like my vadas to be more on the crispy side, so I tend not to pour sauce over the top, but it's up to you, really. You can also sprinkle some paprika and some roasted, powdered cumin on top, for a little extra zing.