The key to a delicious tempura batter is ice cold seltzer water and sizzling frying oil. The main ingredients for tempura batter are cold seltzer water, cornstarch, flour, egg and a pinch of salt. The batter can be used for vegetables and seafood.
To make the tempura batter, combine 1/2 cup of all-purpose flour, 1/2 cup of cornstarch, 1 cup of chilled seltzer, 1 egg and a pinch of salt. Stir the batter to combine. Lumps in the batter are expected and should not be whisked smooth. For the best quality vegetable or seafood tempura, the batter should be made last. Keep the batter in the fridge while batches are frying in the oil.
Before making the tempura, heat at least 1 inch of canola oil to 360 degrees Fahrenheit in a large cast iron skillet. The oil shouldn't be smoking, but it should be sizzling. Saveur magazine suggests adding in 1/8 cup of sesame oil to the pan before frying the seafood and vegetables to add depth to the flavor profile. Allow the oil to return to 360 F before adding the frying ingredients.
While the ingredients are frying, sprinkle a bit of batter over them. This technique makes the fried tempura crunchier. Remove the leftover batter bits from the oil with a slotted spoon.