1 can sweetened condensed milk---------
The classic in-the-can method is simple: Place an unopened can (or do a few at once) of sweetened condensed milk in a pot, well covered with water, and bring the water to a boil.
Keep at a low boil for two to three hours, making sure the water level always stays above the top of the can (if the can isn’t submerged, it may explode).
The longer it cooks, the thicker the dulce de leche will be; after two hours, you’ll be able to drizzle it, and after three hours, it will be thick enough to sandwich cookies.
Make sure the can is thoroughly cool before you open it. Serve over ice cream, cake, etc.