Combine sugar and water in a large heavy saucepan; cook over medium-low heat, stirring often, until sugar dissolves. Increase heat to medium-high, and boil gently, without stirring, until syrup turns a deep amber color, occasionally brushing down sides of pan with a wet pastry brush and swirling pan (about 8 minutes). (Swirling the pan, instead of stirring, promotes more even cooking.) Gradually add cream (sauce will bubble vigorously). Add butter; stir gently until smooth. Remove from heat, and stir in vanilla.