First, remove the label and clean the adhesive from the can of sweetened condensed milk.
Place the can in a pressure cooker and cover with water. Since we want the can to be completely submerged the whole time, have at least two fingers worth of water over the water.
Seal the pressure cooker and set it to pressure mode.
Place on a burner over medium heat and cook until you hear a hissing sound.
Once the hissing starts, set a timer for 20-30 minutes. (The longer you cook it the thicker and darker it will get)
When the 20 minutes are done, turn off the burner and release the valve to allow the pressure to escape.
When the pressure has been released, open the pressure cooker and remove the condensed milk can out. Use tongs or a canning lifter for this since it is very hot.
Do not open the can when it is very hot, It will explode and you can severely burn yourself! Let the can cool in a water and ice bath for an hour or so.
Open the can after it has cooled, use it right away or store it in an airtight container for future use.