The recipe is simple but slow. You put milk and sugar in a wide pan and stir gently over low heat for around two hours. As the water in the milk evaporates, the sugars start to change colour - first pale gold, then amber, then a rich caramel brown. The smell fills the whole house.
Uruguayan kids love alfajores - two soft round biscuits sandwiched together with a thick layer of dulce de leche, often dipped in chocolate or rolled in coconut. They are sold in every supermarket, caf茅 and corner shop, and many children get one in their school lunchbox.
It looks like simple cooking - and it is - but the magic is real chemistry. The slow cooking changes how the sugar and the milk's natural proteins fit together. Scientists call this the 'Maillard reaction', the same thing that gives toast its golden crust.
There is a friendly argument between Uruguay and Argentina about which country invented dulce de leche first. The truth is probably that both invented it at about the same time. Either way, it is now spread (literally) all across South America. In Uruguay you can also find it in ice cream, in cake fillings and even in some breakfast cereals.