Across the border in Bangladesh, the town of Bogura is so famous for its version that 'Bogurar doi' is a household name, and both sides of Bengal claim the dessert.
Milk is boiled down and sweetened with jaggery or sugar, then set to ferment overnight in a small earthenware pot whose porous clay walls slowly wick away moisture, thickening the yogurt into a dense, caramel-toned set custard. Sweet shops across Kolkata sell it by the pot, straight off the fermenting rack.