In a small saucepan, heat cream and corn syrup over medium heat, stirring frequently, just until steaming. (Do not boil.) Pour hot cream mixture over chocolate; let stand for about 2 minutes. Using a silicone spatula, slowly stir small circles in center of bowl until mixture comes together and is shiny and smooth. Stir in butter and vanilla until fully combined. Serve warm or at room temperature. Refrigerate in an airtight container.