In a heavy medium saucepan, combine sugar and water and turn heat to medium-high. Bring mixture to a boil, stirring constantly. When sugar has dissolved, turn heat to high. Cook without stirring until mixture is medium amber. If part of the mixture is turning brown and the other isn’t swirl pan carefully but don’t stir. This process takes about 6 minutes.

Remove from heat. Carefully stir in cream (it will spit and hiss a bit), then stir in the vanilla and salt. Let cool 5 minutes, then whisk in butter.

Serve warm or at room temperature.