Soak the glass noodles in a bowl of warm water for about 20 minutes while you prep everything else. Drain when softened and pliable. Do not boil them yet.
Bring a small pot of water to a boil and blanch the chicken pieces for 1 minute, then drain and rinse. This quick parboil pulls out scum and gives you a cleaner-tasting braise, which is the traditional first step for jjimdak.
Stir the soy sauce, brown sugar, oyster sauce, mirin, garlic, ginger, sesame oil and black pepper together with the water in a measuring jug. This is your braising liquid.
Put the chicken, potato, carrot, onion and dried chilies in a wide pot or deep skillet. Pour the braising liquid over the top, it should come about halfway up. Bring to a boil, then lower to a steady simmer, cover, and cook for 15 minutes.
Uncover and check the potato. It should be nearly tender. Add the drained glass noodles, tuck them under the liquid, and simmer uncovered for another 6 to 8 minutes. Stir now and then so the noodles cook evenly and soak up the sauce as it reduces to a glossy coat.
When the sauce is thick enough to cling and the noodles are soft and chewy, fold in the scallions and kill the heat. Taste and add a splash more soy or a pinch of sugar if it needs it. Scatter sesame seeds over the top and serve.
