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1
Heat the oil in a wok or deep saucepan to exactly 180 °C, confirmed on a probe thermometer. Below 180 °C the noodles fry dense and greasy rather than puffing open; above it they scorch before the centre of each strand has a chance to expand. Hold the temperature steady — this is the single condition every subsequent step depends on.
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2
Divide the dry vermicelli into portions of 20–30 g, so that each batch hits the oil as a loose, even tangle and every strand is exposed to full heat at once. Drop one portion into the oil; within 5 seconds the strands should expand explosively into a puffed, brittle, golden cloud — any yielding or translucency means the oil was too cool. Remove immediately with a spider or slotted spoon, drain on a rack, and allow the oil to return to 180 °C before repeating with each remaining portion.
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3
Hold the fried noodles uncovered at room temperature on the rack, so that any surface steam escapes freely rather than settling back into the crust and softening it. Do not cover or stack them. A single layer on the rack keeps the structure rigid until the sauce is ready.
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4
Combine the tamarind liquid, palm sugar, fish sauce, and lime juice in a clean wok or saucepan over medium heat. Reduce, stirring, until the sauce thickens to an almost caramel consistency — a spoon drawn across the base of the pan should leave a slow-closing channel, and the surface will sheet rather than drip. This is where the dish lives or dies: sauce that is under-reduced carries too much water and will collapse the puffed noodles within 30 seconds of contact, while sauce reduced beyond this point will seize and harden rather than coat.
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5
If using shrimp, add them to the reduced sauce and cook briefly until pink and just firm, then remove the pan from the heat. Keep the sauce at approximately 60 °C as you proceed, because a sauce that is too hot will steam the noodles on contact and begin to soften their crust before you have finished folding.
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6
Add the puffed noodles to the sauce in the wok and fold them through with immediate, gentle movements — three or four deliberate turns using two spoons or chopsticks — so the strands pick up a thin, shiny film without being pressed or broken. The finished noodles should show no pooling of sauce at the base of the wok. Aggressive mixing or prolonged tossing shatters the puffed structure irreversibly.
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7
Transfer at once to a serving plate and arrange the bean sprouts, green onion, and dried tofu over and around the noodles. Serve within 60 seconds of the final toss. At the table the dish is ready when the first bite produces an audible crack — if the noodles yield silently, the window has passed.