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1
Bring 1.5 litres of full-fat milk to the boil in a heavy pan, then remove from the heat and add the lemon juice or vinegar in a thin stream, stirring gently, so the acid distributes evenly and the curds form in soft, uniform flakes rather than ragged clumps. The milk will coagulate into soft white curds separating cleanly from pale green whey. Line a colander with muslin, pour in the curds, and drain for 20 minutes; rinse under cold water to remove the acid, then gather the muslin and press lightly — the finished chhena should be moist but hold its shape when squeezed, not wet, not crumbly.
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2
Pour the second 1.5 litres of full-fat milk into a wide, heavy-based pan and bring to a vigorous simmer over medium heat, using a wide pan so the surface area works in your favour and the moisture drives off steadily without the solids catching. Stir frequently, scraping the sides and base, and continue reducing until the milk reaches one-third of its original volume — a thick, creamy rabri that coats the spoon heavily. The surface will be deeply ivory and the mass will move sluggishly; this stage cannot be shortened without sacrificing the body of the final sweet.
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3
Break the fresh chhena into the reduced milk and stir to combine, then add the crumbled khoya if using. Cook together over low-to-medium heat, stirring without pause, for 10 minutes — the mixture will tighten and the chhena will begin to integrate into the rabri while retaining its granular character. Do not add sugar yet, because premature addition causes caramelisation and darkens the pale cream colour that defines the Alwar preparation.
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4
Add the sugar and continue to stir continuously over low-to-medium heat — this is where the dish lives or dies, as the high milk-solids content scorches in seconds if the spoon stops moving. The correct grain — neither smooth like barfi nor crumbling like dry curd — is achieved only by maintaining patient, constant agitation, so keep the spoon working until the mixture pulls away from the sides of the pan as a single cohesive mass that leaves the base cleanly. When it smells of warm, caramelised dairy and moves as one piece, it is ready to set.
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5
Grease a shallow square tray and transfer the mixture into it, pressing to an even layer roughly 2.5 cm deep, then scatter the crushed pistachios over the surface and press gently so they adhere. Allow to cool completely at room temperature so the grain has time to set and the block firms through to its centre — do not cut while warm, as the mass will crumble. When the surface is dry to the touch and the block feels firm under light pressure, it is ready.
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6
Lay sheets of edible silver leaf over the surface, handling them with care as they cling to any warmth or moisture, then cut into squares with a sharp, clean knife. The cut edge should show a pale cream interior with visible but cohesive grain — smooth-edged cuts indicate under-cooking, and a collapsing edge indicates the block was cut too soon.