Pour the sweetened condensed milk into a glass pie pan and cover tightly with aluminum foil. Set the pan inside the roasting pan and pour hot water around it until it's about half way up the side of the pie pan.
Bake in the preheated oven until it's browned and caramelized, about 1 to 1-1/2 hours. Check the caramel at the 1 hour mark, just use some tongs to carefully lift up one corner of the foil. If the milk still looks pale, leave it in for another 15 minutes or so. Remove from the oven, remove the foil, and allow the caramel to cool on a wire rack. Whisk or blend to smooth it out. Store in the refrigerator in a sealed container for up to 1 month.