Preheat the oil in a small frying pan over medium heat. Add in the annatto seeds and fry for about 2 to 3 minutes and keep stirring to avoid burning the seeds. Remove from the heat and strain the oil into a glass container and discard the seeds.
Cook noodles (if using store-bought banh canh):
Cook the banh canh or udon noodles according to the package instruction. Set aside
Preheat a large pot with 1 Tbsp annatto oil. Add the shallots, and garlic and saute for about 1 minute until fragrant. Add crab paste and stir fry for another minute. Add the shrimp and stir fry until they turn pink. Remove the shrimp pieces to set aside for topping later and transfer the rest to the soup below when ready
Pour chicken stock in a large pot. If you are using crab claws, add the claws in when the stock comes to a boil and cook until the crab claws are cooked through
Add the shallots, garlic, and crab paste you sauteed earlier into the soup and bring to a boil. If you are using crab meat, this is the time to add it in along with the shrimp balls. Add seasonings and adjust to your liking by adding more fish sauce and/or sugar if needed
If using fresh banh canh, cook the noodles in the soup. The noodles will float to the top when cooked and thickening the soup at the same time. Continue to ladle the noodles and soup into bowls and continue to assembling
Thickened the soup (skip if using fresh made banh canh):
Mix the tapioca flour with water and stir until mixture has no lumps and smooth. While the soup is gently simmering, slowly pour the tapioca mixture into the soup while the other hand keeps stirring. The soup will thicken and shiny because of the tapioca flour
Portion the noodles out into an individual serving bowl. Portion the shrimp, shrimp balls, crab meat or crab claws into each bowl
Ladle the thickened soup over the noodles. Garnish with chopped green onions and cilantro leaves. Few turns of fresh ground black pepper is a great addition too. Serve immediately
The annatto seeds oil will give the soup its redish color without affecting much of the taste. It's commonly used in Mexican cuisines too.