Cut the chicken thighs into bitesized pieces, with the skin on and place in a large bowl.
Add the grated ginger, sake, mirin, white pepper, mayonnaise, and soy sauce. Stir to combine so that everything is evenly coated. Cover in plastic wrap and refrigerate for 4-24 hours to marinate.
Combine the potato starch with a little salt in a bowl
Take out as few pieces of chicken and toss in the potato starch to cover completely. Gently shake off the excess and arrange on a plate, continue with more pieces.
Pour in around 600ml of oil into a medium saucepan and heat over a medium temperature until it reaches around 175–180°C (345–355°F).
Add a few pieces of chicken, one at a time - not too many to crowd the pan as you'll reduce the temperature too much and create greasy karaage. For a medium saucepan (18-20cm/7-8") around 8-10 pieces will be good.
First Fry - Fry the chicken for 5-6 minutes until browned and crisp. Remove and drain on paper towels. Continue with the remaining chicken pieces until complete.Note: the first fry can be done up to 5 hours in advance, just store the chicken in airtight containers in the fridge.
Second Fry - After a minimum of 10 minutes of resting, return the chicken to the oil (again at 175–180°C (345–355°F)) and fry for 2-3 minutes to recrisp. Drain once again on paper towels.
Serve the Karaage with a small salad and lemon wedges and a little helping of Japanese mayonnaise to dip.
Notes
Make it Gluten-FreeUse tamari instead of light soy sauce. Potato starch is naturally gluten-free.Serving & StorageBest served hot or warm. You can first-fry up to 5 hours ahead, chill, then second-fry to crisp.Leftovers can be eaten cold or reheated in the microwave (crispness will soften).Freeze after the first fry; second-fry from frozen in hot oil for about 4 minutes, until piping hot.