Heat 8 TB of the butter, brown sugar, and salt in a large saucepan over medium heat until small bubbles appear around the perimeter of the pan, 4 to 8 minutes. whisk in the cream and cook until a ring of bubbles reappears, about 1 minute. Off the heat, whisk in the vanilla. Transfer the hot frosting mixture to a bowl and, with an electric mixer on low speed, gradually mix in the confectioners sugar until incorporated. Increase the speed to medium and beat until the frosting is pale brown and just warm, about 5 minutes. Add the remaining 4 TB butter, 1 piece at a time, and beat until light and fluffy, about 2 minutes.