In a medium saucepan over medium heat, combine chocolate, butter and milk. Bring to a boil and let boil 1 minute. Stir in sugar and salt until dissolved. Heat, stirring constantly, to between 234 and 240 degrees F (112 to 116 degrees C), or until a small amount of syrup dropped into cold water forms a soft ball that flattens when removed from the water and placed on a flat surface. Remove from heat and stir in vanilla. Let cool 10 minutes.
Beat fudge with a spoon until it loses its gloss. Pour quickly into the buttered dish. Refrigerate 30 minutes, until firm.