Set a large pot of water on the stove to boil. Preheat oven to 425°F (218°C).
Add the sweetened condensed milk to an 8-inch square baking dish (or 9-inch pie plate) and spread around into one even layer. Cover the baking dish tightly with foil and place it in a large roasting pan.
Fill the roasting pan with the hot water you set on the stove to boil earlier until it reaches halfway up the sides of the baking dish.
Place the roasting pan in the oven and bake at 425°F (218°C) for 1 hour and 30 minutes to 1 hour and 45 minutes or until the mixture is thickened and it turns an amber color. Make sure to check the roasting pan about halfway through the baking time and add more hot water if needed.
Remove from the oven and carefully remove the baking dish from the roasting pan. Place on a wire rack and carefully remove the foil from the dish. Whisk the dulce de leche until smooth, then allow to cool completely.
Once cooled, transfer the dulce de leche to an airtight container and store in the refrigerator.
Notes
Leftover dulce de leche can be stored in an airtight container in the refrigerator for up to one week.Sweetened condensed milk: You can use any brand of sweetened condensed milk for this recipe, just avoid using the fat-free kind.