Place butter in 9 inch square baking pan. heat at 350 for 4-5 minutes or until melted; sprinkle with brown sugar. Arrange bananas in pan; set aside. prepare cake mix according to package directions; pour over bananas.
Bake at 350 for 30-35 minutes or until a toothpick inserted near the center comes out clean. Immediately invert cake onto a serving platter. Let stand for 5 minutes; remove the pan. Serve warm with whipped cream.