To make the dipping sauce, combine soy sauce, vinegar, water, sugar, fresh chilies, and toasted sesame seeds in a small bowl. Set aside.
Put potatoes and onion into the blender, blend them until you get a puree. (Do not process too long) Pour the potato puree in a mixing bowl and add the potato starch and mix well. If you can't find potato starch, use rice flour or cornstarch instead.
Heat a tablespoon of oil in a pan over medium-high heat until very hot. Pour in a ladle-full of pancake batter. You should hear the sizzling sound immediately, and the batter will bubble right away on the edges. Using the ladle, quickly spread out the batter thinly.
Cook for 2-3 minutes. When the edges turn deep golden brown and crisp, flip the pancakes to the other side and cook for another 2-3 minutes. Add more oil if needed. Repeat the frying pancakes with the rest of batter.
Served pancakes hot with the dipping sauce.