Bring the apple cider syrup and the brown sugar to a boil over medium heat.  Boil for three minutes. Whisk in the heavy cream and bring it to a boil or one minute. Reduce the heat to low and whisk in the butter one tablespoon at a time.  When all the butter is incorporated quickly stir in the salt and then pour the sauce into a mason jar while still hot. Allow it to cool. Store covered in the fridge.