1. Combine milk and chocolate in medium-size heavy saucepan; cook over low heat until chocolate is melted. Add sugar, corn syrup and salt and cook, stirring constantly, to boiling.
2. Cook, without stirring to 234F on a candy thermometer. (A teaspoonful of syrup will form a soft ball when dropped into cold water.) Remove from heat at once. Add vanilla and butter or margarine, but do not stir in.
3. Cool mixture in pan to 110F, or until lukewarm; beat with wooden spoon until mixture thickens and begins to lose its gloss. (This will take about 15 minutes.) 4. Spread in a buttered 8x8x2" pan. Let stand until set and cool; cut into squares. Makes about 2 pounds.