Technically, butterscotch is a hard candy, a somewhat less-cooked form of toffee that dates back to the early 19th century. Over time, the term has come to describe sauces, ice creams, and puddings that feature brown sugar and butter as the primary flavors.
In a 2- to 3-quart saucepan over medium-high heat, bring the brown sugar, corn syrup, and butter to a simmer, stirring continuously. Simmer for about 5 minutes, then add the cream and salt and bring back to a simmer. Cook 3 more minutes, stirring all the while. Serve hot or cold.