Combine 2 teaspoons of grated ginger, grated garlic, soy sauce, cooking sake, sesame oil, salt, and pepper in a small bowl.
Coat the chicken with the marinade in a large bowl and mix thoroughly by hand.
Cover the chicken and let it marinate in the fridge for at least 1 hour.
Mix the Kewpie mayo and 1 tablespoon of grated ginger in a bowl.
Refrigerate the ginger mayo until ready to serve. (If desired, add a squeeze of Sriracha for spice.)
Fill a heavy-bottomed pot or skillet 1 inch deep with neutral oil and heat.
On a large plate, make a pile of all-purpose flour and another with potato starch.
Coat each piece of marinated chicken first in the all-purpose flour, then in the potato starch, making sure to dust off the excess.
Line a wire rack with a couple of pieces of paper towel to absorb excess oil from the fried karaage.
Once the temperature reaches 325 F, the oil is ready for frying. (Test by inserting a wooden chopstick into the hot oil to see if it creates small bubbles.)
Add 4 to 5 pieces of chicken at a time, being careful not to crowd the pan.
Deep fry the karaage for about 1 ½ minutes or until it is a pale golden color before transferring it to the paper towel-lined wire rack.
After all the chicken has been fried once, raise the oil temperature to 350 F for a second fry.
Return each piece of chicken to the pan and fry for 1 minute or until the exterior is golden brown in color.
Serve the ginger karaage hot with freshly-squeezed lemon and a side of ginger mayo.