Bake this cake up to two months in advance and let it cool; seal it in two layers of plastic wrap and freeze it. Thaw the wrapped cake at room temperature for two hours.
To make sauce in microwave, place butter in 4-cup microwavable measuring cup. Microwave uncovered on High about 1 minute or until melted. Stir in brown sugar and corn syrup. Microwave uncovered on High 2 to 3 minutes or until sugar dissolves, stirring once halfway through cook time. Stir in whipping cream. Microwave uncovered on High 45 to 60 seconds or until mixture boils, stirring once halfway through cook time.