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1
Submerge the dried spur chillies and dried bird's eye chillies in warm water and leave for 15 minutes, so that the flesh softens all the way through and the skins become supple enough to break down under the pestle. Lift them out and squeeze firmly to expel excess moisture. The chillies should feel pliant but not wet, because a paste made from insufficiently dried chillies will be loose and will resist fat-release during cooking.
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2
Place coriander seed and cumin seed in a dry pan over a low flame, stirring continuously so the heat reaches every seed evenly and none catch and scorch on one side. Toast until the seeds smell warm and nutty — not sharp or acrid — then remove immediately and grind to a fine powder. Scorched cumin turns bitter and will dominate every other element in the paste.
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3
Begin pounding in the mortar in the following order, working each addition to a near-paste before introducing the next: salt, lemongrass, galangal, squeezed chillies (both varieties), shallots, garlic — building from the toughest fibrous material to the softer aromatics so each ingredient is fully broken before the next makes the mass wetter and harder to reduce. Add the ground coriander and cumin, then work in the kapi. This is where the dish lives or dies: pound with patience and sustained force until the paste is almost entirely smooth, with no visible chilli skin fragments remaining and the mass cohering as a single dense, deep-red compound, because a rough paste will never integrate properly into hot fat.
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4
Test the finished paste by pressing a small amount between two fingers and drawing them apart slowly. The paste should stretch and pull cleanly without fibrous threads or visible skin shards, confirming that the cell walls of every aromatic have been fully ruptured and their oils released into the mass. It is ready when the surface appears uniformly fine-grained and the colour is an even, deep brick-red.