Melt butter in a cast-iron skillet or heavy saucepan set over medium-high heat. Add brown sugar and salt, stirring until sugar is completely melted. Transfer sugar mixture into mixing bowl and, using an electrical mixer on low speed, beat in vanilla. Add confectioners' sugar, about 1/2 cup at a time, alternately with heavy cream, beginning and ending with confectioners' sugar. Mix until well blended after each addition, and scrape down sides of bowl as needed. Continue beating until frosting becomes spreadable. If too thick, add a little more cream; or, if too thin, add a little more confectioners' sugar.