Remove the label from the can of sweetened, condensed milk and, using a can opener, make 2 tiny holes (opposite each other) in the top of the can. Place the can in a large pan and carefully pour in the boiling water, allowing the water level to reach three quarters of the way up the sides of the can.
Place the pan over high heat to allow the water in the pan to come to a rolling boil again, then turn the heat down and simmer for 3 hours. During this time, some of the sweetened, condensed milk will come out of the can as the can cooks. This happens because, as the can heats, the milk inside it expands.
After 3 hours, take the pan off the heat and, using a pair of thongs or a slotted spoon, pluck the (hot!) can out of the pan. Leave to cool to room temperature before opening. Stir until smooth.
Cooled dulce de leche can be stored in the fridge in an airtight container for up to 3 weeks.
Notes
Dulce de leche that has been stored in the fridge may need to be warmed through in the bowl of a double boiler before use. As it cools, it thickens, but it will become runny again when heated.
Recipe by The Tough Cookie at https://thetoughcookie.com/2014/01/19/making-dulce-de-leche-made-in-the-can-open-can-method/