In the bowl of an electric mixer fitted with the flat beater, beat the butter and salt of medium speed until light and fluffy, 2 minutes. Reduce the speed to low, gradually add the confectioners sugar and beat until encorporated. Add the vanilla and beat until smooth and creamy. stop the mixer and scrape down the sides of the bowl. Add the chocolate and beat on medium speed until incorporated, stopping the mixer occcasionally to scrape down the sides of the bowl. Do not overmix or the frosting will incorporate too much air; it should be creamy and dense, like ice cream. Add milk, 1 T at a time if frosting does not have spreadable consistency.