Preheat the oven to 425°F and arrange a rack in the middle.

Pour the condensed milk into a pie plate and sprinkle with salt. Cover tightly with aluminum foil and place in a roasting pan. Place the roasting pan in the oven and add enough hot tap water to reach halfway up the sides of the pie plate. Bake for 30 minutes to one hour. Keep checking to make sure the water is still there.

Remove the roasting pan from the oven and remove the pie plate. Carefully uncover the plate and whisk the mixture until smooth, for about 1 minute. Replace the foil and return the pie plate to the water in the roasting pan. Place back in the oven, adding more hot water so that it remains halfway up the sides of the pie plate the entire time. Continue to bake until the dulce de leche is dark golden brown (about the color of peanut butter), for about 1 1/2 hours more. Keep checking the water every 30 minutes or so.

Remove from the oven and transfer the pie plate to a wire rack. Carefully remove the foil and whisk the dulce de leche until smooth, for about 3 minutes. Let cool to room temperature, then refrigerate in an airtight container with a tightfitting lid for up to 1 week.