In a 1-½-quart heavy saucepan, mix together ⅓ cup of condensed milk, corn syrup, butter, and salt. Cook over medium heat, stirring constantly, until mixture reaches 235 degrees; F on a candy thermometer (soft-ball stage). Stir in remaining milk. Stir in chocolate, a square at a time. Boil until syrup reaches 235 degrees F again. Remove and stir in vanilla. Pour into an 8x2-inch buttered pan. Cool and cut into half-inch squares.