In the container of a blender, place chocolate, butter, and port.

In a medium saucepan, combine evaporated milk, sugars, and salt. Cook over medium heat, stirring constantly, until sugars and salt are dissolved. Add to blender. Let stand for 5 minutes.

Process until thick and smooth, about 15 seconds. Using a spatula, stir mixture to check consistency. If needed, blend in additional 15-second increments to thicken more.