Bring a large pot of water to a boil. Once boiling, add the cauliflower florets and boil for about 4 minutes, or until the cauliflower is almost cooked. Do not overcook.
Drain and rinse under cold water to stop it from cooking more and to cool it.
Heat about 2 cups of oil in a deep saucepan over medium heat. In the meantime, prepare the tempura batter.
Whisk together the flour, cornstarch, salt, curry powder, and baking powder in a mixing bowl. Pour in the chilled soda water and whisk to combine, do not overmix, it's okay if there are some lumps.
Test the oil by dipping a wooden spoon or chopsticks into it. If bubbles appear around it, the oil is hot enough.
Working with one cauliflower floret at a time, dip it into the tempura batter (using a pair of chopsticks or a fork) and slightly shake it to remove the excess batter. Immediately transfer to the oil and repeat with 3-4 more florets. Fry for about 1 minute, or until golden brown. Remove the cauliflower tempura from the oil using a slotted spoon and transfer to a plate lined with a kitchen paper towel to remove the excess oil.
Repeat with the remaining cauliflower florets, frying no more than 4-5 at a time.
Serve immediately with the ginger sauce on the side! Or you can pour some of the ginger sauce into a deep plate and top with the cauliflower tempura. Cauliflower tempura just stays crispy a few minutes so it's best served immediately.