Mix the pork mince, chopped leek, grated ginger, pinch of salt, sake, potato starch, soy sauce and one egg with your hands until entire mix is sticky. Roll out into golf ball sized balls. Then fry in 170 deg oil until gold. Place on kitchen paper to dry. In a small bowl, mix all the ingredients for the sauce then add to a pot to heat. Stir constantly to prevent the sauce from going hard. Then put the balls in the pot and coat well with the sauce. In my picture, I have served with pak choy boiled in salted water. See http://sumokitchen.com/JapaneseRecipes/black-vinegar-meat-balls/ for full recipe.