Place the sliced red chillies in a bowl and add the soy sauce. Set aside. For the broth, pour the chicken stock into a deep pan.
Add the chicken cubes, cinnamon, spring onions, ginger, sugar and salt and bring to a boil over high heat.
Lower heat and simmer for forty-five minutes, skimming the scum off as it rises to the surface. Add the fish sauce and remove from heat.
Remove the chicken with a slotted spoon and set aside to cool. Strain the broth and keep warm over very low heat.
Boil plenty of water in a deep pan. Add the noodles and blanch for about five minutes or until they soften. If using vermicelli it will take around two minutes.
Drain and rinse in cold water. Drain again thoroughly and set aside. When the chicken cools completely, shred into thin strips.
Place the noodles/vermicelli in individual serving bowls, top with bean sprouts, shredded chicken and onion. Pour the hot broth into the bowls.
Sprinkle with white pepper powder; garnish with the basil and chopped fresh coriander.
Serve piping hot, with the lemon wedges and the bowl of red chillies in soy sauce on the side.