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1
Wrap the kapi in foil and set it directly over a low flame or in a dry pan until it smells toasted and slightly smoky, about 2–3 minutes per side, then unwrap and allow to cool — toasting drives off raw ammonia and concentrates the paste's savour, so it deepens the curry rather than dominating it. Place the white peppercorns and salt in the mortar and pound to a fine, uniform powder; no whole peppercorns should remain visible. The salt acts as a grinding medium here, giving the peppercorns something to break against so the powder comes fine and even.
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2
Add the cooled kapi to the mortar and pound it into the pepper powder until fully amalgamated — the paste must be cool, or its moisture will turn the dry powder to a slick that resists further work. The mixture should be dry and sandy in texture, with no separate lumps of shrimp paste. This dry, gritty base is the foundation every subsequent aromatic is built into.
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3
Add the galangal and pound steadily, working in a circular motion against the mortar walls until the galangal is fully broken down into fibrous threads and then to paste — pressing the fibres against the curved wall rather than just striking them downward tears them apart far more efficiently. Add the lemongrass in two additions, reducing each to fine paste before adding the next, because lemongrass is stubborn and a large mass will simply bounce rather than break down. Continue until no distinct fibres remain; run your fingers through the paste to check.
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4
Add the kaffir lime rind and pound until incorporated and no longer distinguishable as separate pieces — the rind carries bitter, floral oils that need to be fully ruptured and married into the paste rather than left as pockets of raw sharpness. The paste at this stage should be coarse-smooth with a strong citrus-bitter aroma. Taste: if you can still identify individual pieces by texture, keep pounding.
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5
Add the shallots and pound to paste, then add the garlic and reduce likewise, so their sharp, raw edge is worked out and they become sweet and cohesive within the mass. The paste will become noticeably wetter as the alliums release their moisture; continue pounding until no raw chunks remain and everything coheres into a single, even body. Resist the temptation to add the chillies before this stage is complete.
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6
This is where the dish lives or dies: add the fresh green chillies — spur chillies first, bird's eye second — in small additions, pounding each addition to complete smoothness before introducing the next, because all preceding aromatics must already be at fine-paste consistency; if they are not, the flood of moisture from the chillies will prevent the fibrous elements from ever breaking down properly. The paste should turn a deep, brilliant emerald green as the cells rupture and release their pigment. Do not use a blender at any stage — blending aerates the paste, turning it pale and producing a waterlogged, raw-tasting result that no amount of cooking will fully correct.
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7
Add the krapao or horapha leaves and pound briefly until fully broken down and the paste is uniformly fine and deeply coloured — work quickly, because prolonged pounding of fresh herbs at room temperature causes the green to dull and grey as the volatile fragrance dissipates. The finished paste should be brilliant emerald green, almost smooth, cohesive, and intensely fragrant, with no stringy fibres and no separate pools of liquid. Use immediately, or press cling film directly against the surface and refrigerate for up to three days.