This story is promulgated in a popular guide book and has become something of an urban legend. It is likely that the unique origin of Cao lầu in Hội An, is due to the town's history as a trading port. Some incorrectly state that prior to the Chinese establishment in Hội An, in the 17th century, the town was also a centre of Japanese trading activity. The famous Hội An bridge (built by the Japanese) also dates to this period. Therefore, some have speculated that Cao lầu noodles could be derived from Japanese soba noodles. However, this theory is flawed, because Cao lầu doesn't contain buckwheat flour.
Also, the bridge was built as a symbolic gesture of peace, connecting the Chinese and Japanese quarters, so both trading merchant groups were present in Hội An at the same time. Japanese udon noodles is another possibility, but udon doesn't use lye water. On the other hand, some Chinese wheat noodles are similar to Cao lầu (made with rice flour), because both are lye water kneaded noodles (an innovation that originated in ancient southern China). The dish usually includes sliced pork Xa Xiu (Cha siu), another Chinese influence. There are many variations of this dish in Hội An, some vary the way the pork is cooked (or omitted), using different greens (or absence of mint), sometimes topped with fried pork rind, peanuts, rice crackers, and/or scallions; and sometimes served with lime or chili jam. In the last few decades, new restaurants with modern versions have added shrimp or chicken, and additional herbs. And the pork broth is sometimes blended with dried shrimp/squid or chicken stock. The only consistent item are the noodles. So if you see Cao lầu noodles, then you have Cao lầu.
In Hội An, Cao lầu restaurants typically have two levels, no air conditioning and various red and green lanterns hanging as decoration. If you want to eat Cao lầu, you will have to go to the second floor of the restaurant. Cao lầu differs from typical Vietnamese noodle dishes because it has no soup. In Vietnamese, locals call it a "mixing dish" because it includes vegetables, fried lard and sauce on top of the noodles. The ingredients are placed in the dish, but it's the customer who mixes them together. Cao lầu is therefore a special variety of noodle dish.
It is also different from Quang noodle, another Vietnamese noodle dish, because of the amount of sauce, the additional ingredients, and the type of noodles used. To make Cao lầu noodles, the rice has to be stone ground and mixed with ash and water. The ash is made with firewood from the Cham Islands, around 19 km from Hội An. The noodles are cut and then cooked three times with firewood. The water to cook the noodles is also very special because it only comes from specific wells in Hội An. This is why Cao lầu is a dish that can only be prepared in Hội An. Cao lầu combines various flavors (sour, pungent, bitter, astringent and sweet) in the vegetables, soy sauce and fried lard. Cao lầu is served at room temperature.
According to VietKings (Kyluc.vn)