Peel the bananas and cut them into slices or strips. Mix all the dry ingredients in a bowl, add cold water and stir until the batter reaches a thick consistency. If the batter is too runny, add a little flour.
Heat the oil to 350 F. Dip each piece of banana into the batter. Dredge the banana slices in the panko, poured on to a plate or bowl, until completely coated.
Fry bananas in hot oil for 3-4 minutes until they turn golden or orange. After this, transfer the tempura bananas to napkins to absorb excess oil. Serve with powdered sugar or sweet sauce.