First, take a mixing bowl, crack the egg, put it in the bowl, and beat or mix it well.
Take chilled or ice-cold water in a cup and gradually add in the beaten egg while mixing.
After adding the water, add flour by gradually spreading over the mixture. Then, add cornstarch, salt, and baking powder to it.
Whisk all the ingredients in the mixing bowl. But remember, don't overmix them because the lumps in the batter are not a problem, and they help make them crisp.
Let the bananas peel off and cut according to your desired shape. For this recipe, I'm cutting some in ½ thick pieces and some in length-wise halves.
Heat the oil in a deep fryer to 350-375°F (175-190°C).
Dip each banana slice into tempura batter, coating it completely and evenly from all sides.
Carefully place all these batter-coated banana slices in hot oil. Only a few at a time, not to overcrowd the fryer. After 2-3 mins or until golden brown, take them out with the help of tongs.
Put these fried batter-coated banana tempura on the paper towel to drain excess oil. Repeat the process until all are cooked.
Serve the banana tempura warm, either on its own or with optional toppings such as powdered sugar.