Melt chocolate in milk in a heavy saucepan over low heat, stirring constantly. Stir in sugar and salt. Increase heat to medium; bring to a boil, stirring constantly. Continue to boil without stirring until a small amount of the mixture forns a soft ball* in cold water, or about 234° on a candy thermometer. Add butter and vanilla then cool to lukewarm, or about 110°. Beat until fudge loses its gloss then quickly pour into a pie plate or loaf pan. Cool and cut into squares. Recipe can be doubled.
*To Test for Soft Ball Stage
A small amount of syrup dropped into chilled water forms a ball, but is soft enough to flatten when picked up with fingers (234° to 240°).