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1
Drain the soaked black glutinous rice and steam over boiling water for 40–50 minutes, turning the rice once at the halfway point so the grains on the outside do not overcook while those in the centre remain hard. Steam until every grain is fully tender and translucent through to its centre. Test by pressing a grain between the fingers: it should offer no chalky resistance.
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2
Spread the cooked rice in a thin, even layer on a clean tray or board, leaving it uncovered so moisture escapes evenly rather than condensing back into the rice. Leave at room temperature until completely cool — not merely warm but genuinely cold to the back of the hand. This is where the dish lives or dies: any residual heat above ambient temperature when the starter is added will kill the yeast and lactic bacteria, and the fermentation will not proceed.
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3
Crumble the look pang evenly over the cooled rice and fold through with a clean spoon or hands, working deliberately so the starter reaches every part of the mass and leaves no dry pockets, since an uneven distribution will give you patches that ferment and patches that do not. The rice should show a faint, starchy, faintly yeasty smell at this stage — no sourness yet.
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4
Pack the inoculated rice firmly into a clean, dry, airtight vessel, pressing out air pockets as you go so the grains sit in close contact and the starter can travel through the mass evenly. Seal completely with minimal headspace, because oxygen at the surface encourages off-flavours rather than the clean, sweet fermentation you are after.
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5
Ferment the sealed vessel at 30–35°C, a temperature warm enough for the starter to work at a steady pace without driving the fermentation so fast that harsh, vinegary notes develop before sweetness has fully built. At 3 days, open and assess: the rice will have softened further and a sweet, slightly alcoholic liquid will have accumulated around the grains, and the aroma should be clean, sweet, and faintly lactic. For a mildly sweet result, stop here; for a more complex, sour flavour with greater alcoholic depth, reseal and continue for a further 2 days, tasting at intervals as the sweetness diminishes and acidity builds.
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6
Serve the fermented rice at room temperature or lightly chilled, with its accumulated fermentation liquid poured over. The liquid is sweet, slightly cloudy, and drinkable alongside.