Cut 400 g chicken breast into large bitesize pieces.
Add 2 tsp soy sauce, 1 tsp ginger root, 1 tsp garlic, 1 tsp sake and 1 tsp sesame oil to a mixing bowl, mix well and add the chicken pieces. Mix until the chicken is fully coated, cover with plastic wrap and rest in the refrigerator for 30 minutes. (I also recommend measuring out the water for the batter at this point and chilling for 30 minutes too!)
Start heating your oil to 170 °C (338 °F). While you wait, gently whisk 1 medium egg(s) with 50 ml water, being careful not to let it become too foamy.
Sift 4 tbsp all-purpose flour and 3 tbsp potato starch into the bowl and mix gently, it's okay if there are small lumps in the batter.
Once the oil is preheated, dip the chicken in the batter and deep fry for 4 minutes, turning it halfway through.
Transfer the cooked toriten to a wire rack to allow the excess oil to drain off.
Mix the 1 tbsp kabosu juice (or citrus of your choice), 1 tbsp soy sauce and 1 tbsp rice vinegar in a small bowl.
Dish up and enjoy!