For the prawn stock: in a saucepan saute the prawn heads and shells in a little oil until caramelised. Cover with 1 1/2 litres water and simmer for 8 minutes. Drain and reserve liquid, discarding shells.
For the chicken stock: place chicken, onion, cinnamon stick, cloves and star anise in another saucepan and cover with the rest of the water. Bring to boil and simmer for another 8 minutes. Drain and reserve liquid. Shred the chicken meat and set aside, discarding the rest of the solids.
Combine the prawn stock and chicken stock in a saucepan. Add the palm sugar and salt to taste and simmer on low heat.
To make the chilli paste, put the chillies, a little water, shallots and garlic into a blender and grind until fine. Heat oil in pan and saute the chilli paste until fragrant; add a pinch of salt. Set aside.
Get a pot of boiling water ready. Blanch the following ingredients one after the other: prawns, yellow noodles, rice vermicelli, bean sprouts, water spinach. As each is just done, drain with a slotted spoon and set aside.
To serve: in deep bowls, put a portion of yellow noodles, rice vermicelli, water spinach and bean sprouts. Ladle the hot stock over the noodles. Top with some chilli paste, shredded chicken, 2-3 cooked prawns and garnish with fried shallots and hard-boiled eggs (optional).