Pour two 14-ounce cans sweetened condensed milk into a 9-by-13-inch baking dish. Cover with foil and set in a large roasting pan; add enough hot water to come halfway up the sides of the dish. Bake at 425 degrees F until slightly reduced and dark amber, about 1 hour 15 minutes. (A spotted skin may form on top.) Transfer to a food processor and puree until the skin dissolves and the dulce de leche is smooth. Chill until thickened, about 1 hour.
Recipe courtesy of Food Network Magazine