For this recipe you will need a double boiler. You can make a makeshift double boiler with a saucepan and a bowl that is placed over the pan. For more details, check out my post on 'Faking a Double Boiler' under 'Basics and Tips'.
Fill the bottom part of the double boiler, or a saucepan if you're faking a double boiler, with 5 cm (or 2 inches) of water. Allow to come to a simmer.
Open the can of sweetened, condensed milk and pour the contents of the can in the bowl of the double boiler. Cover the bowl of the (makeshift) double boiler tightly with aluminum foil.
Place the bowl with the milk over the simmering water and leave to cook for 3 hours, stirring occasionally (as in: every half hour or so) with a rubber spatula to ensure even cooking.
Once the sweetened, condensed milk has caramelized, take off the heat and allow to come to room temperature with the foil still in place. Once cooled, stir until smooth.
Store in an airtight container in the fridge for up to 3 weeks or use immediately.
Recipe by The Tough Cookie at https://thetoughcookie.com/2014/01/21/making-dulce-de-leche-double-boiler-method/