8cupswaterfor water bath, you may need more or less depending on the size of your pan; only use it to fill the roasting pan as indicated.
Make sure your roasting pan will fit in the center rack of your oven (adjust racks if needed) and preheat oven to 425F (220C).
Pour condensed milk into a glass, oven-proof 9.5-10" pie plate¹. Cover tightly with foil, but don't wrap the foil below the pie plate (make it easy for yourself to lift the foil after cooking to check if the dulce de leche is ready, you don't want the foil touching the water or difficult to remove).
Place the pie plate in a roasting pan and add enough water so that the water is about ½" above the level of the condensed milk (you do not want the water to get into the pie plate, but it should be higher than the level of milk).
Transfer to the center rack of your preheated oven and bake on 425F (220C) for 2-3 hours. Check at 2 hours, and if the dulce de leche is not dark enough, add another 30 minutes, then check again and add another 30 minutes if desired (see post for a visual of how my dulce de leche usually looks at each point). Keep in mind it will continue to thicken as it cools.
When desired color and thickness is reached, remove from oven and whisk the dulce de leche to remove any lumps (it may seem quite lumpy at first, just keep whisking!). Allow to cool completely, then use as desired!
¹Alternatively you could use a glass, heatproof pan in any of the following dimensions; 8x8", 9x9", 7x11", 9x13", or any other dimension that fits nicely into your roasting pan.Please see post for instructions on storing.