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1
Shell and devein the prawns, then pat them thoroughly dry with kitchen paper, so that no surface moisture dilutes the tamarind paste and weakens the cure. Even a light film of water is enough to thin the paste and compromise the acid concentration across the whole batch.
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2
In a bowl, combine the thick tamarind paste, sea salt, dried chili flakes, and palm sugar, mixing until the palm sugar has dissolved completely and the mixture is uniform. The paste should be dense and clinging — if it runs freely off the spoon, the acid concentration is insufficient to cure the flesh properly, and the result will be a marinade, not a pickle.
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3
Add the prepared prawns and toss thoroughly until every surface of every prawn is coated. Pack the mixture into a sealed container, pressing firmly to eliminate air pockets so the paste maintains full contact with the flesh throughout the cure, then refrigerate.
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4
This is where the dish lives or dies. Cure for 2–3 hours: at 1 hour the exterior of each prawn will have turned opaque and firm; at 2–3 hours the full prawn should be semi-opaque — the exterior snapping cleanly, the interior still carrying a faint translucency when held to light, the colour shifted from grey-raw to a pale acid-whitened tone. Do not extend the cure beyond 4 hours, because at 6 hours the texture turns mealy and the flavour flat, and there is no recovery.
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5
Remove the prawns from the container and rinse briefly under cold running water for no more than 20 seconds, so the sharp edge of excess salt and acid recedes on a fingertip taste without stripping the cure from the flesh itself. Do not soak — a prolonged rinse leaches the tamarind from the outer layer and dulls the pickle.
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6
Arrange the cured prawns on a serving plate or banana leaf, scatter sliced shallot over the top, and set halved calamansi alongside. The finished prawns should read tart-saline on first contact, with a clean prawn sweetness emerging mid-palate and the chili arriving at the finish.