To make the leek sauce: Pierce the leek randomly with a sharp knife and then chop finely. This method makes it easier to mince.
Mix the soy sauce, sake, rice vinegar, and sugar together in a bowl. Heat the oil in a skillet and lightly saute the minced leek and the red chili. Then add the soy sauce, the sake, rice vinegar,and sugar to the pan, stirring constantly. When the sugar has dissolved, take off the heat and set aside
Pierce the chicken skin with a fork, cut an large pieces in half, and marinade for 5 minutes in the soy sauce and sake
Remove the chicken from the marinade and coat thoroughly with potato starch
For the best result, the chicken should be at room temperature before deep-frying
Deep-fry the chicken at a medium-high temperature (340F). Make sure each piece is completely immersed in the oil so it cooks on the inside while remaining crispy on the outside
Remove when golden brown. Place on a serving dish and cover with the leek sauce