Soak 12 bamboo skewers (6" long) in water for 30 minutes.
Make Tsukune Yakitori. Mince ginger root finely. Chop long white onion finely. Combine the ground chicken, onion, 1/2 tsp salt, Sake, ginger root, and potato starch in a bowl and mix well. Divide into quarters.
Oil hands. Take 1/4 chicken mixture and make an oblong shape. Skewer the meat with 2 bamboo sticks. Make three more, and refrigerate if needed (to cook later).
Sprinkle 1/2 tsp salt on the skewered Tsukune chicken before cooking. Cook chicken on a grill or in a frying pan with a little oil until the meat is cooked through.
Make Chicken Tender Yakitori. Cut a strip of chicken tender into 4 pieces. Skewer chicken pieces on bamboo sticks, 4 pieces per stick.
Remove the pits from Umeboshi plums and chop the plums to make a paste. Slice Shiso leaves very finely.
Sprinkle salt on skewered chicken before cooking. Cook chicken on a grill or in a frying pan with a little oil until meat is cooked through. Top with Umeboshi plum paste and Shiso leaves.