Pour two 14-ounce cans sweetened condensed milk into a 9-by-13-inch baking dish. Cover with foil and set in a roasting pan; add enough hot water to come halfway up the sides of the dish. Bake at 425 degrees F until thick and dark amber, about 1 hour 15 minutes. Transfer to a food processor and puree until smooth. Refrigerate until thickened, 1 hour. (Makes about 2 cups.)