milk and sugar stirred over low heat for hours until the lactose browns and the whole pan turns to a thick amber caramel
Origin: Buenos Aires & the Río de la Plata
From the journey of Sugar Cane.
Dulce de leche is milk and sugar and nothing else that matters, cooked slowly until it thickens and browns. The colour and the flavour do not come from caramelised sugar in the ordinary sense but from the Maillard reaction between the milk's lactose and its proteins, which is why it tastes of milk rather than of burnt sugar and why it cannot be hurried past a certain point without turning grainy. Argentina and Uruguay both claim it and neither will concede. The story told most often on the Argentine side is that in 1829, during negotiations between Juan Manuel de Rosas and Juan Lavalle at Cañuelas, a maid left a pan of lechada, sweetened milk, on the fire while Lavalle slept, and returned to find it brown and thick. The tale is repeated in Argentine schoolbooks and is almost certainly folklore: sweetened reduced milk is far older and far more widespread than one afternoon in Buenos Aires province, appearing as manjar blanco in Peru and Chile, cajeta in Mexico where it is made with goat's milk, and doce de leite in Brazil. What is not in dispute is that Argentina and Uruguay eat more of it per head than anywhere else and have built a great deal of their baking on it. It is the filling of the alfajor, the layer in a chocotorta, the middle of a facturas pastry, and the thing spread on toast at breakfast, and Argentine cooks distinguish grades with some care: repostero is stiff enough to pipe and to hold in a cake, clásico is looser and is what goes on bread. The bicarbonate of soda in the recipe is not optional and is not a shortcut. A small amount raises the pH, which speeds the Maillard reaction considerably and darkens the pan without needing a higher heat, and higher heat is what scorches the milk solids on the bottom.
Use the widest pan in the kitchen and keep the heat low; the whole difficulty of dulce de leche is that it needs two and a half hours and will scorch in thirty seconds. The bicarbonate of soda raises the pH and speeds the browning, so do not leave it out, and add it off the heat because it foams. Judge doneness by colour and by the spoon trail rather than by the clock, and stop earlier than looks right since it firms up as it cools. The boiled-condensed-milk shortcut gives something sweeter, paler, and less milky, and Argentine cooks do not accept it. It splits if you stir a cold spoon through a hot pan, so use one wooden spoon throughout.
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