Bring 75 ml sake, mirin, soy sauce, sugar and ginger syrup in a small sauce pan to a boil and let it simmer fora bout 5 mins on low heat. Stir regulary. Add one or two tablespoons of the mixture to a bowl and add the chicken pieces. Let it sit for about 5 minutes or so.
Put a piece of chicken and a piece of spring onion on a skewer. Finish the skewer by adding three more pieces of chicken and two more pieces of spring onion to it. Repeat this for the other skewers.
Preheat a grill pan. Put the marinated skewers in the hot grilling pan and turn them every minute. Before turning brush them with some of the marinate sauce from the bowl. Keep turning and brushing until the chicken is cooked and golden brown. (should take 4 to 6 minutes).
Serve with plain rice, noodles or as a snack. Sprinkle some fried onions over the yakitori and serve with the sauce you kept in the sauce pan.