Stir sugar and 1/2 cup water in heavy large saucepan over medium-low heat until sugar dissolves.

Increase heat; boil without stirring until syrup turns golden (I advise to not wait until it turns amber, as that usually means it's burnt!), occasionally brushing down sides of pan with pastry brush dipped into water and swirling pan, about 12 minutes.

Remove from heat. Whisk in butter. Gradually add cream (mixture will bubble vigorously). Stir over low heat until smooth. Cool to lukewarm before serving.