Cut pork belly into large (0.75” x 1.5”) chunks. Bring a large pot of water to a simmer and blanch the meat just until it turns opaque. Drain, rinse, and set aside.
Peel the arrowhead roots. Just trim off the top and bottom, and peel the outer skin. Immediately drop each peeled root into a bowl of cold water to prevent oxidation. If using potatoes, you're going to want to cut them into 1 1/2-inch chunks, but don't cut them just yet (they take less time to cook than the arrowroot).
When you’re ready to cook, take the arrowroots out of the bowl of water and cut each in half. Heat the oil in a wok over low heat and add the rock sugar. Let it melt slowly and then add the fermented bean curd/liquid. Stir for a minute. Now add the blanched pork belly and arrowhead roots (if using potatoes, do not add them yet).
Turn the heat to medium and mix everything well. Add the shaoxing wine, dark soy sauce, and 2 cups of water. Stir everything together and cover the lid. Simmer over medium low heat.
Check the pot every 5-10 minutes. As the liquid evaporates, add more water 1 cup at a time and replace the lid. Continue doing that until the meat is fork-tender and the sauce is thickened enough to coat the pork. In my case, it took 5 cups of water, and about 1 hour and 15 minutes to fully cook the pork and arrowhead root. If using potatoes, cut them into chunks and stir them in after the pork has cooked for about 45 minutes.
If the pork is fork tender, but there’s still a lot of visible liquid, uncover the wok and turn up the heat to medium high. Stir constantly until the sauce is reduced to a gravy-like state.