In a small pan simmer together the yakitori tare ingredients until slightly thickened, about 5 minutes. Set aside. Also at this point put your skewers in a tray of water to soak.
Remove the chicken thighs from the bone and mince everything with a sharp knife. Chop until you get something slightly more rough than ground.
Add the chicken thigh mince to a bowl along with the egg, potato starch, green onion, shallot, and a pinch of salt. Knead everything together for quite a while, around 10 minutes should do it. Treat it like kneading bread, you want to knead for so long the mixture is sticky-tacky.
Preheat your oven to 225 Celsius. Oil your hands and shape the mixture into elliptical cylinders. Run them through with two skewers and place on a silicone mat lined baking tray.
Place the tray with the tsukune on the topmost rack of the oven and bake for 10 minutes. Remove, flip, and brush with the yakitori tare sauce. Place back in the oven on a middle shelf and bake for another 10 minutes. Remove, flip, brush with more yakitori tare, and place back on the topmost shelf for 5 minutes, or until the sauce begins to caramelize.
Serve these tsukune with a raw egg yolk and some more chopped green onion. You can also sprinkle a bit more coarse salt right on top.