Pour the sweetened condensed milk into a shallow glass pan, like a pie dish. Cover the dish tightly with foil.
Place the wrapped pan into a larger baking dish. Fill up baking dish with hot water until it is about half as high as the small baking pan. Put in the oven.
Cook for 45 minutes in the preheated oven. Check the water level in the pan, if its gotten too low, replace with more hot water. Continue cooking for 45 minutes.
Remove from oven. Once cooled, whisk it until it is smooth and store in an airtight container in the fridge.