Combine the chicken, soy sauce, sake, ginger, garlic and sugar and refrigerate for 30 minutes.
Drain off any pooling marinade, add the potato starch and toss until every piece carries a thick, uneven white coating.
Rest the coated chicken for 8 minutes while heating 1 1/2 inches oil in a medium heavy pot to 350°F.
Lower one-third of the chicken into the oil and fry for 4 to 5 minutes, turning once, while keeping the oil between 335°F and 350°F.
Remove the batch when the crust is lightly golden and a large piece reads 165°F.
Drain on a rack, return the oil to 350°F and repeat with the remaining chicken.
Let the karaage stand for 4 minutes before serving so escaping steam does not burn anyone.