Soak the bamboo skewers for at least 30 minutes. Peel and grate ginger and set aside. Cut and separate the green and white parts of the green onions. Cut the green parts of the green onions into 2-inch, put handful of the green parts in a saucepan. Thinly slice the white parts of the green onion and set aside. In the saucepan, add soy sauce, mirin, sake, water, and brown sugar. Bring the sauce mixture to boil over medium high heat. Once boiling, reduce the heat to low and simmer, uncovered, until the liquid is reduced by half (about 30 minutes). Let sauce cool.
Heat non-stick frying pan over medium heat. Add 1/3 of ground chicken and break it up into small pieces. On a large bowl, put in white parts of the green onion, cooked chicken and the rest of the raw chicken. Add sesame oil and knead (with spatula) the mixture until it becomes sticky, pale in color. Grease an oven safe wire rack as it supports the chicken skewers well while the excess oil drips down to the baking sheet. Divide the mixture into 8 equal portions, then coat your hands with sesame oil. Scoop 1/8 of the mixture onto your hand and toss it to release air pockets. Form the mixture into a 4-inch-long oval patty and insert the skewer into the patty. Place the skewer on the rack and lightly sprinkle salt over. Preheat the oven to high broil (550F) for 5 minutes. Put an aluminum foil over the skewers and broil for 7-8 minutes. Then flip the skewers and broil for another 5-6 minutes. After both sides are cooked, brush the sauce on both sides of the meat and broil for another 1-2 minutes. Transfer the skewer onto plate and brush more sauce on it.