1 can (15 ounces) sweetened condensed milk
1 cup light corn syrup
1 tablespoon butter
1/8 teaspoon salt
2 ounces (2 squares) unsweetened chocolate
1 teaspoon vanilla extract
In a 1-1/2-quart heavy saucepan, mix together 1/3 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 8×2-inch buttered pan. Cool and cut into half-inch squares.