Peel and cut potatoes into ⅛-inch slices. If possible, use a mandoline to get uniform slices. (I open my Benriner mandoline to nearly the thickest slicing capacity.) Put slices in a bowl and cover with water.
Put a large pot of water on to boil over a high heat. While waiting for it to come to a boil, whisk together the sauce in a large bowl: chili oil, soy sauce, vinegar, Sichuan pepper oil, sugar, sesame oil and chicken powder or msg (if using).
When water is boiling, add potato slices. The boil will die down, and by the time it just recovers to a full boil the potatoes are probably close to done. I look for about a ¼-inch "ring" of doneness around the edges, with the interior still crisp. Depending on the thickness of your potatoes, you may need another minute or so, but they need to remain somewhat firm so they don't fall apart in the sauce. Transfer the potatoes to a colander and drain well.
Immediately add the potato slices to the sauce bowl about ⅓ at a time. Use tongs to make sure each slice gets fully covered with sauce on both sides before transferring to a serving plate. Finish with remaining potatoes, ⅓ at a time. When all slices are nicely arranged, spoon any remaining sauce on top. Garnish with the scallions and serve at room temperature.
