Stir together the water and sugar in a medium-size saucepan with a clean spoon.
Simmer over medium heat without stirring until the sugar caramelizes into a deep amber color. Remove from the heat and add the butter. Be careful, because the butter will create a lot of steam. Allow to sit until the butter is melted, then stir until smooth.
Whisk in the heavy cream, and season with salt to taste.
Cool slightly before serving.