Add apple cider to medium sauce pan and bring to boil. Boil for about 30 minutes or until the cider has reduced to ⅔ cup.
Reduce the heat and add the butter, cream and brown sugar. Bring back to boil and then reduce heat and simmer for about 10 minutes or until the temperature has reached 212°F. The caramel should be a dark color and thick.
Remove from heat and stir in salt. If the caramel thickens too much, place over low heat and add cream to thin.
Don't skip on reducing the apple cider.
You can replace light brown sugar with granulated sugar.
A thermometer isn't necessary but can be helpful in determining when sauce is ready.