Start by blending the soaked dried red chillies, ground coriander, fennel, cumin, turmeric, ginger, and half of the chopped shallots in a food processor. Add a splash of water if needed to help blend everything into a smooth paste.
In a large pot, heat the cooking oil over medium heat. Add the remaining chopped shallots and sauté until golden brown and fragrant, about 5 minutes.
Add the blended spice paste to the pot and stir well. Cook the paste for about 5-7 minutes until the oil begins to separate and the mixture is aromatic.
Next, pour in the chicken stock and prawn stock, followed by the cinnamon stick. Bring the mixture to a gentle simmer.
Add the mashed sweet potatoes and tamarind concentrate to the pot. Stir to combine and let it simmer for another 10-15 minutes.
Once the broth has thickened, stir in the coconut milk and allow it to simmer for another 5 minutes. Season with salt to taste.
While the broth is simmering, prepare your yellow noodles according to package instructions and blanch the beansprouts.
To serve, place a portion of yellow noodles in a bowl, ladle the hot broth over the noodles, and top with beansprouts, hard-boiled eggs, fried shallots, and any other toppings you desire.