Sift the flour, baking powder and cornstarch into a medium bowl. Stir in 1 cup of the soda water, the salt, and cayenne. Allow to rest for a few minutes. (Additional soda water may be needed to adjust thickness).
Dip your diced veggies into the mixture to coat them. Fry in oil heated at 375* F until golden brown, about 3-5 minutes.