Add the 4 grams konbu and 200 grams low-sodium umeboshi to a heavy-bottomed pot with a lid and then arrange the 1200 grams chicken drumettes around them, so the ume are evenly distributed, and there aren't many air-gaps between the pieces of chicken. Add the ½ cup mirin and cover the pot with a lid.
4 grams konbu, 200 grams low-sodium umeboshi, 1200 grams chicken drumettes, ½ cup mirin
Bring the mixture to a boil over high heat and then turn down the heat to low. Set a timer for 20 minutes and let the chicken braise undisturbed.
After the timer goes off, open the lid and taste the braising liquid. If it's not salty enough, add the 1 tablespoon usukuchi soy sauce. If it's already well seasoned, you can leave it out. Redistribute the chicken, so the pieces that were not submerged in the braised liquid are submerged.
1 tablespoon usukuchi soy sauce
Continue cooking the chicken over low heat for another 10 minutes or until the wings are tender. If the liquid starts running low, you can add a little water to keep it from burning.
When the chicken is tender, remove it from the pot and turn up the heat to reduce the sauce into a glaze.
When the braising liquid has thickened to the texture of maple syrup, return the chicken to the pot and toss to glaze.