One of the most frequently recommended ways to make Dulce de Leche is to put a can of sweetened condensed milk into boiling water and cook it for a period of 2-4 hours. This is not a good thing to do – and one look at the warning on a can of sweetened condensed milk will tell you why: the can can explode. This can happen when the water level in the pan gets to low, when the burner you’re using has a hot spot below your can, or when any other situation arises in which excess heat builds up in that can. I’ve used the method with good results – but it’s not worth the risk of stepping out of the kitchen and returning to a big mess of sweetened condensed milk everywhere, not when there is a safer method that is just as easy to do and that is even a bit faster.
This method for making dulce de leche uses a double boiler. Simply open a can of sweetened condensed milk and pour it into the top of a double boiler, where the bottom is already filled with a few inches of simmering water. With a double boiler, you don’t need to worry about the water evaporating while you cook, and since there is no pressure from a sealed can, there is no risk of an unexpected explosion (yes, it really does happen).
You can make your own double boiler with a large bowl and a saucepan, if necessary. The condensed milk will heat up and thin slightly, and as it cooks it will take on a caramel color and the more caramelized flavor of dulce de leche. Stir the mixture every once in a while to keep it smooth and remove it from the heat after about 60-70 minutes. The dulce de leche will thicken as it cools. If you like yours very thick and caramel-like, you can keep cooking it for an extra 15-30 minutes. Store the finished dulce de leche in an airtight container. It can be refrigerated, but I will often keep it at room temperature if I am planning to use it up within a couple of days.