Melt butter in a 3-qt. saucepan over medium heat. Add brown sugar; bring to a boil, stirring constantly. Stir in whipping cream and vanilla; bring to a boil. Remove from heat, and let cool 1 hour. Transfer to a mixing bowl.
Sift powdered sugar into icing. Beat at high speed with an electric mixer until creamy and spreading consistency.
Fix It Faster: This impressive cake also works well as a 2-layer cake if you don't want to split the layers. You'll have some frosting leftover, but that was okay with us!