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1
Drain the soaked dried chillies and pound them in a mortar with the fresh turmeric, ginger, shallots, garlic, and coriander root until a rough paste begins to form, working in this order so that the harder, drier aromatics break down before the wetter ones turn the paste slick and difficult to pound. Add the shrimp paste and the dry-roasted, ground coriander seed, cumin, and cardamom, then continue pounding, incorporating the lemongrass last so its fibres have the least time to tangle and resist. The paste is ready when it is cohesive, deeply coloured amber-orange, and uniformly textured with no large fibrous fragments remaining.
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2
Heat a large heavy pot or wok over medium-high heat, add the thick coconut cream, and cook, stirring steadily, until the fat separates visibly — the surface will shimmer and the cream will begin to fry rather than simmer, so that the paste added next will fry in that rendered fat rather than stew in liquid and lose its depth. The smell shifts from sweet to toasted and the texture becomes oily and glossy; this cracked state is what you are after before the paste goes in.
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3
Add the khao soi paste to the cracked coconut cream and fry, stirring constantly, for 3–4 minutes, so that the fat carries the heat evenly into the paste and draws out the fat-soluble colour and warmth of the turmeric, dried chilli, cardamom, cumin, and coriander seed in a way that liquid alone never could. This is where the dish lives or dies: the paste must cook long enough that its raw edge disappears and its warm spice blooms fully into the oil, but not so long that it scorches and turns bitter. The paste is ready when it is deeply fragrant, a shade darker than when it entered the pot, and the fat has taken on a clear amber-orange colour.
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4
Add the chicken legs, turning them in the paste to coat on all sides and colour them briefly — 1–2 minutes — so the paste clings to the skin and seasons the meat from the outside as it braises. Pour in the thin coconut milk and chicken stock in equal measure (400 ml each), add the fish sauce and palm sugar, and bring to a gentle simmer rather than a rolling boil, so the coconut milk stays smooth and does not split into a grainy broth. Cook uncovered for 25–30 minutes until the chicken is cooked through and the broth is deep amber-orange — darker than a red curry, with a warm yellow cast from the turmeric — tasting deep and warm, with the paste's spiced complexity in the foreground and the coconut richness behind it, not sweet or immediately bright with lemongrass-galangal notes.
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5
Heat neutral oil to 180 °C in a wok or deep saucepan; at this temperature the noodles set and puff within seconds rather than sitting in the fat long enough to turn greasy. Fry a handful of the reserved raw egg noodles in the hot oil — they will puff, stiffen, and turn golden within 60 seconds — then remove with a spider or slotted spoon and drain on a rack. The finished noodles should be rigid, evenly golden, and audibly crisp when pressed.
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6
Bring a large pot of water to a full boil and cook the remaining fresh egg noodles for 2–3 minutes until fully tender — cooked to complete softness, not al dente, so they absorb the broth rather than sitting stiffly against it in the bowl. Drain well so that excess water does not dilute the broth when they are ladled over, then divide between four warmed bowls.
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7
Ladle the broth and one chicken leg over the noodles in each bowl, then arrange the crispy fried noodles on top as a crown immediately before service, so the steam and broth do not soften them before they reach the diner. Set a lime wedge, a tablespoon of pickled mustard greens, and a few slices of raw shallot alongside each bowl so the diner can adjust the balance of brightness, funk, and sharpness to their own taste. Place a teaspoon of nahm prik pao at the table for each diner.