Clean and chop 1 banana flower (vazhaipoo) into small pieces. Soak them in buttermilk for 10 minutes to remove bitterness. Drain and set aside.
Soak 200 grams of chana dal (Bengal gram) in water for 2 hours. Drain the water and grind the dal coarsely without adding any water.
In a mixing bowl, combine the chopped banana flower, coarsely ground chana dal, 1 finely chopped onion, 2 finely chopped green chillies, a handful of chopped curry leaves, a handful of chopped coriander leaves, 1/2 teaspoon of grated ginger, 1/2 teaspoon of fennel seeds, and salt to taste.
Mix all the ingredients well to form a thick batter. If the batter is too dry, sprinkle a little water to adjust the consistency.
Heat oil in a deep frying pan over medium heat. Wet your hands and take a small portion of the batter, shape it into a round ball, and flatten it slightly to form a vada.
Gently slide the vada into the hot oil and fry until it turns golden brown on both sides. Fry the vadas in batches, making sure not to overcrowd the pan.
Remove the fried vadas from the oil using a slotted spoon and place them on a paper towel to absorb excess oil.