Cut each chicken thigh into 2-inch pieces and season with salt and freshly ground black pepper.
Grate the ginger (you will only need ½ tsp) and mince the garlic (I love this garlic press).
In a large bowl, combine the ginger, garlic, soy sauce, sake, and sesame oil. Whisk it all together.
Add the chicken to the bowl and mix it with your hands. Cover and keep in the refrigerator to marinate for 30 minutes.
Pour the oil into a heavy-bottomed pot (I used a Dutch oven) and heat it to 325ºF (163ºC) on medium heat.
Meanwhile, prepare the potato starch and all-purpose flour in separate piles.
First, lightly dredge each marinated chicken piece in the flour and dust off the excess flour. Then, dredge it in the potato starch and remove the excess starch.
Continue with the remaining chicken pieces.
When the oil temperature has reached 325ºF (163ºC) (you’ll know it’s ready when you insert a wooden chopstick into the oil and small bubbles appear around it), gently submerge each chicken piece in the oil. Do not overcrowd the pot; add only 3-5 pieces at a time. If you put too many pieces in at once, the oil temperature will drop quickly, and the chicken will end up absorbing too much oil.
First Deep-Frying: Deep-fry for 90 seconds, or until the outside of the chicken is a light golden color. If the chicken browns too quickly, then the oil temperature is too high. Either put a few more pieces of chicken in the oil or lower the heat. Controlling the oil temperature at all times is very important for deep-frying. Transfer the chicken pieces to a wire rack to drain the excess oil.
The residual heat will continue to cook the chicken as it rests on the wire rack. Continue deep-frying the remaining chicken pieces. Between batches, pick up and discard the crumbs in the oil with a fine-mesh sieve. This keeps the oil clean and prevents it from becoming darker.
Second Deep-Frying: Now, heat the oil to 350ºF (177ºC). Place 3 to 5 pieces of the resting chicken back into the oil and deep-fry for 45 seconds, or until the skin is golden brown and crispy. Transfer them to a wire rack to drain the excess oil. Continue with the remaining chicken pieces.
The left photo shows the chicken pieces after the first frying and the right photo shows them after the second frying. You can see the chicken pieces on the right are slightly darker in color.
Serve the chicken hot. Karaage is often served with a wedge of lemon and Japanese mayonnaise.Sprinkle shichimi togarashi for a bit of spice, if desired.
Deep-fry all the chicken pieces, let cool completely, and keep in an airtight container. You can store it in the refrigerator for up to 3 days and in the freezer for up to a month.