Over medium to high heat, melt the butter, brown sugar, vanilla, salt, and cream. As soon as the mixture starts to boil, set a timer for 1 minute. Boil for one minute only! Remove from heat, let cool about 3 minutes, then sift in 1 cup powdered sugar. Stir briskly with a whisk. This icing sets up as it cools, so you need work fast. While still warm, spread the icing over the baked bars. Let cool, cut into bars, and serve. See notes for helpful tips.