Combine chocolate and butter in top of a double boiler; place over boiling water, and cook, stirring constantly, until chocolate and butter melt. Add sugar, salt, and milk. Cook, stirring constantly, until sugar dissolves and mixture is slightly thickened. Remove mixture from heat, and stir in vanilla. Serve sauce over ice cream.
Note: Sauce may be refrigerated in a covered container, and reheated in top of a double boiler.