Pound the chicken breasts with a meat tenderiser or the back of a knife until ¼-1/3 inch (5-7mm) thick. Then cut into pieces (roughly 1x3 inch or 2 x 7cm)
Mix the garlic, ginger, soy sauce, sake, and white / black pepper in a bowl. Add the chicken and leave to marinate while you prepare the batter.
Pour the oil into a pan or deep-fat fryer and heat to 180C or 350F while you prepare the batter.
You'll need two mixing bowls for the tempura batter one for the batter and the other to contain icy water to keep the batter chilled. This is the secret to get it to crisp up nicely. Pour 1-2 cups of water into one bowl with water and ice. Place the other bowl on top. There should be enough water to touch the bottom of the second bowl.
Sift the cornstarch/cornflour, all purpose flour/plain flour into the bowl and whisk together.
Whisk in the egg, then pour in the chilled sparkling water and whisk until a smooth batter is formed.
Add the marinated chicken pieces to the batter, using kitchen paper to remove any excess marinade first so extra moisture from the marinade does not alter the consistency of the batter.
Add no more than 4-5 pieces of batter-coated chicken to the hot oil one at a time using tongs. Glide each one through the hot oil for a few seconds to get the best texture on the batter and fry for 2-4 minutes until crisp and slightly golden.
Remove from the pan and set aside on a wire rack.
Repeat with the remaining chicken and serve immediately with ponzu dipping sauce.