Tempura flour is available through Asian food stores,
if not available it can be replaced with half cup of flour,
half cup of cornflour and two teaspoon of baking powder
(instant heat).
It is important that iced water is used this prevents
the batter absorbing too much oil and from becoming sticky.
it is made to order and used immediately as it is a fragile
batter and will hold and be at its peak for 5 minutes
the batter should still contains flour lumps, this will
give the mottled, bumpy effect that tempura dishes have,
it is best achieved with by mixing the batter with chopsticks
you do not coat ingredients with the batter too much.
This batter while always associated with Japanese cuisine
was actually introduced to them by the Portuguese in the
16th century. The origin of the word tempura is due to
Portuguese missionaries that ate fish due to the Catholic
proscription against meat during Lent, in Latin, "ad
tempora cuaresmae", means "in the time of Lent".