In a medium saucepan, combine the brown sugar and butter. Cook over moderate heat, whisking occasionally, until the butter has melted and the sugar has dissolved, 3 to 5 minutes. The mixture will bubble—lower the heat, if necessary. Add the chocolate and salt and whisk until the chocolate is melted and the sauce is smooth. Add the cream—the sauce will bubble up. Continue cooking, whisking frequently, until the toffee sauce has thickened slightly, about 5 minutes. Remove from the heat and stir in the vanilla.