First, prepare the dipping sauce. Combine the rice vinegar, soy sauce, brown sugar, grated ginger, and sliced chilli, and mix well to dissolve the sugar. Set aside.
Next, prepare the vegetables. Cut the cauliflower into small florets, trim the ends off the tenderstem and remove any leaves, and cut the mushrooms into halves or quarters.
Combine the plain flour and cornflour in a large bowl with some salt and pepper.
Begin to heat up an inch or two of oil in a large, deep pan or wok over a high heat. When the oil is nearly ready, finish making the batter by adding the sparkling water to the flour mixture. Mix to combine, but don't over mix it, as you don't want to beat out the bubbles - it doesn't matter if a few lumps remain.
Working in small batches, coat the vegetables in the batter, and drop them into the hot oil. Cook for several minutes, turning the vegetables over halfway, until the batter is lightly golden. Transfer the cooked vegetables to a plate lined with paper towels while you cook the next batch.
Repeat with the remaining batter and vegetables. Season with more salt if desired, and serve with the dipping sauce.