In a small bowl, combine cream, milk, and cornstarch; set aside. In a medium saucepan, combine sugar, corn syrup, and butter. Cook over medium-high heat, stirring, until butter has melted and sugar has dissolved, 4 to 5 minutes.
Reduce heat to medium-low. Add milk mixture; bring to a boil. Boil 30 seconds; remove from heat. Cool slightly before serving, or store, covered, in the refrigerator up to 2 weeks. Before serving, reheat in a saucepan; thin, if necessary, by adding up to 2 tablespoons cream.