Combine sugar, corn syrup, and water in a large, light-colored saucepan with high sides. Cook over medium heat, swirling occasionally, until mixture is amber in color, 15 to 20 minutes. Reduce heat to low, and very carefully whisk in reserved cream-butter mixture; stir constantly as caramel mixture will bubble up and steam. Continue cooking over low heat, stirring occasionally, for 5 more minutes. Remove from heat. Set out 1/2 cup of caramel sauce for the cake, and reserve the remainder for the topping. Allow to cool to room temperature.