If there are any blood spots, yellow fat, cartilage, or tough tendons on chicken thighs, remove them. Then, cut the meat into bite-sized pieces, about 1-1.5 oz (30-40 g) each, and transfer them to a bowl.
In the bowl with the chicken, first add salt and sugar, and massage them in well. Then, add sake, grated garlic, grated ginger, and soy sauce, and mix everything together again. Let it sit at room temperature for 20-30 minutes to allow the flavors to meld.
In a separate bowl, combine flour and potato starch (or cornstarch or Japanese rice flour) and mix well. Add the seasoned chicken to this bowl and coat it evenly with the flour mixture. Let it sit for about 5-10 minutes to allow the coating to adhere properly to the chicken.
Heat oil in a pot until it reaches 320-340℉ (160-170℃). Once the oil is hot, carefully add the chicken pieces one at a time. Deep-fry them for about 3 minutes.Be careful not to add too many pieces at once, as this can cause the oil temperature to drop significantly. The chicken should occupy no more than about two-thirds of the oil’s surface area.If you don’t have a cooking thermometer, you can check the oil temperature by dropping a small piece of batter into the oil. Remove the chicken pieces from the oil and let them rest on a wire rack for at least 3 minutes. If there are any fried bits left in the oil, skim them out with a mesh skimmer or strainer. Repeat steps 4 and 5 until all the pieces are fried.
Increase the heat and raise the oil temperature to 360-380℉ (180-190℃). Then, carefully add the chicken pieces back into the oil one at a time and fry them for about 1 minute, or until golden brown. As before, avoid adding too many pieces at once.The 1-minute frying time is just a guideline. If the pieces are smaller, they will cook faster, and if they are larger, it will take longer. Once the chicken turns golden brown or starts floating to the surface of the oil, that’s a sign it’s done. Be careful not to overcook it—frying for too long will make the chicken dry and tough instead of juicy. Remove the chicken pieces from the oil and let any excess oil drain off on a wire rack. If there are any fried bits left in the oil, skim them out with a mesh skimmer or strainer, then repeat steps 6 and 7 until all the pieces are fried.
Serve the karaage on a plate. Feel free to garnish it with a lemon wedge and fresh vegetables of your choice.