For the stovetop method: Remove the label from the can of sweetened condensed milk. Place 3 to 4 forks upside down on the bottom of a large 6- to 8-quart pot. Place the unopened can on top of the forks and cover with water by a few inches. (The forks help to prevent a vacuum between the bottom of the can and the pot.) Place the pot over high heat until it reaches a simmer, then lower the heat to medium-low or low to maintain a steady simmer for about 3 hours. Add more water to the pot if necessary to ensure that the can is submerged at all times. Let the can cool to room temperature in the water before removing. Open the can and scoop out the contents. The dulce de leche should be dark and caramelized.
For the oven method: Preheat the oven to 425 degrees F. Place a shallow baking dish in a roasting pan. Pour the sweetened condensed milk into the baking dish and cover the dish with foil. Transfer the roasting pan to the oven and add enough hot water to reach halfway up the sides of the baking dish. Bake until the dulce de leche is dark and caramelized, 60 to 75 minutes, checking the water level occasionally and adding more if needed. Let cool, then whisk until smooth.
Transfer the dulce de leche to an airtight container, put plastic wrap directly on the surface and refrigerate for up to 2 weeks.