In a large saucepan with tall sides (because the mixture will bubble), combine the sugar, corn syrup and 1/4 cup water. Heat over medium-low heat until the sugar dissolves, stirring occasionally. Increase the heat to medium-high and bring to a boil without stirring. Boil to a deep amber color, 5 to 6 minutes.
Remove the pan from the heat and whisk in the cream (be careful ¿ the caramel will bubble up vigorously!). Whisk in the vanilla, butter and salt. Let cool slightly, then transfer to a glass jar with a lid. (The caramel sauce will keep for 3 to 4 months in the refrigerator.)