Why this recipe works
Mishti doi is two sciences at once. First, fermentation: a spoon of live curd seeds the warm milk, and the bacteria sour and set it, the same culture as everyday dahi. Second, caramel: the jaggery (or browned sugar) adds deep, toffee notes through the Maillard reaction. Setting it in an earthen pot lets the clay wick away water for that signature thick, fudgy body.
Ingredients
Method
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1
Simmer the milk down until reduced and slightly thickened.
Why: reducing concentrates the milk solids so the set doi is thick and rich rather than watery.
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2
Melt the jaggery to a caramel and stir it into the warm milk until dissolved.
The colour and depth: caramelising the jaggery browns its sugars (Maillard and caramelisation), giving mishti doi its toffee colour and flavour.
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3
When the mix is just warm (not hot), whisk in the live curd starter.
Why just-warm: too hot and you kill the culture; too cold and it will not set. Blood-warm is the sweet spot for the bacteria to work.
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4
Pour into an earthen pot or bowl, cover, and leave somewhere warm 6 to 8 hours until set. Chill before serving.
An honest word
Mishti doi is a sweet, and it is sugar-rich, so it is a treat, not an everyday health food. That said, it is a better sweet than most: it is fermented, so if you do not overheat the culture it carries some live bacteria, and it uses jaggery over refined sugar. Enjoy it as dessert in a sensible portion, and it earns its place.