Start by slicing the chicken breasts into strips. Aim for uniform thickness, about 1 inch wide, so they cook evenly. In a bowl, combine the sliced chicken with the soy sauce, salt, and pepper. Allow it to marinate for at least 15 minutes.
In a separate large mixing bowl, add the flour, cornstarch, baking soda, and the whisked egg. Gradually pour in the soda water while gently whisking to combine. The batter should be lumpy but not overly thick.
In a deep fryer or a Dutch oven, heat the vegetable oil over medium-high heat until it reaches 350°F (175°C). You can test the oil by dropping a small amount of batter in; if it sizzles and rises to the surface, it’s ready.
Dip each marinated chicken strip into the batter, ensuring it’s well-coated. Carefully lower it into the hot oil, one or two pieces at a time to avoid overcrowding. Fry until golden brown, about 4-5 minutes, turning occasionally with tongs.
Serve your Chicken Tempura hot, with your choice of dipping sauces. It pairs wonderfully with rice or can be enjoyed on its own as a delightful appetizer.