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1
Pour 1 litre of whole milk into a wide, heavy-bottomed pan, set over a medium flame, and bring to a simmer, stirring frequently so the milk does not scorch and turn bitter on the base. Reduce slowly, scraping the sides and base of the pan, until it reaches approximately 750 ml — 75% of its original volume — because a concentrated milk gives the finished doi its dense, creamy body rather than a loose, watery set. It is done when a spoon drawn through the milk leaves a brief, clear trail before the liquid closes; stir each skin that forms back into the milk as it appears.
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2
While the milk reduces, place the sugar in a separate dry, heavy pan over a medium flame and allow it to melt at the edges without stirring, then tilt the pan gently to encourage even melting, so that stirring does not cause the sugar to crystallise and seize. Cook to a light amber caramel — the colour of pale honey — because a deeper brown will introduce bitterness that no amount of milk can soften. It is done when the caramel smells of butterscotch and holds a clear, bright amber colour.
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3
Remove the caramel pan from the heat and pour the hot reduced milk into it in a thin, steady stream, stirring constantly so the violent bubbling is controlled and the caramel dissolves evenly rather than setting hard. The caramel will seize momentarily and then dissolve completely into the milk; return the combined mixture to a low flame and stir until any remaining lumps are fully incorporated. This is where the dish lives or dies: insufficient caramelisation produces a pale, merely sweet yoghurt with none of the amber colour or caramel-milk depth that defines mishti doi.
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4
Remove the sweetened, caramelised milk from the heat and allow it to cool, monitoring the temperature with a thermometer until it reaches exactly 40–42°C, because above 45°C the culture is destroyed and below 38°C it will not work vigorously enough to set the yoghurt. It is done when the milk feels barely warm — blood heat — against the inside of the wrist. Do not rush the cooling by chilling the pan, which risks an uneven temperature throughout the milk.
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5
Whisk the starter in a small bowl until smooth, then stir in two tablespoons of the warm caramelised milk to loosen it, so the culture disperses into an even suspension rather than dropping in cold clumps that set unevenly in the pot. Pour the thinned starter back into the remaining milk and stir gently but thoroughly to distribute the culture without beating in air, which would disrupt the calm, undisturbed environment the culture needs to set. It is done when the mixture is uniform in colour and consistency with no visible streaks of starter.
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6
Ladle the mixture equally into four unglazed earthenware matkas and cover each with a small square of muslin or its own lid, then place the pots in a warm room completely undisturbed for 8 hours, because any movement during setting breaks the fragile structure forming within the yoghurt and produces a grainy, weeping result. The unglazed clay draws whey slowly through its walls so the yoghurt firms and concentrates without refrigeration, giving mishti doi its characteristic dense texture and matte, amber surface. It is done when the pot can be tilted slightly without the yoghurt moving.
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7
Once set, serve the mishti doi directly from the earthenware at room temperature, or move the pots to a cool place to firm the texture further before serving. It is done when a spoon pressed lightly to the surface meets clean, dense resistance and withdraws leaving a sharp, defined impression.