Wash 200g of urad dal thoroughly. Soak in plenty of water for 4-5 hours until the grains swell up and break easily when pressed.
Drain the dal completely through a colander. Place the drained dal in the refrigerator for 30 minutes to chill, which prevents the blender from heating the batter.
Grind the chilled dal in batches, adding only 1-2 tablespoons of ice-cold water at a time. It should turn into a thick, pale ivory paste.
Transfer batter to a bowl. Whip vigorously with your hand in a circular motion for 5 minutes until it is airy, light, and passes the water float test.
Fold in the rice flour, chopped green chilies, minced ginger, torn curry leaves, crushed pepper, cumin, hing, and salt until evenly combined.
Wet your hands. Take a portion of batter, shape into a ball on your palm, flatten slightly, and make a hole in the center with your thumb.
Gently slide the shaped vada into medium-hot oil. Fry on medium heat, turning occasionally, until the exterior is deep golden-brown and the sizzling noise quietens.