Butter 9 inch square pan. Combine in heavy 3 quart saucepan milk and chocolate. Stir over low heat until chocolate is melted. Do not allow mixture to boil. Stir in remaining ingredients. Cook slowly until sugar is dissolved. Increase heat to medium and bring mixture to boiling. Cook, stirring occasionally, to 234 degrees (soft ball stage). Remove mixture from heat and cool to lukewarm (110 degrees) without stirring or jarring. Then add 1/4 cup butter and 4 teaspoons vanilla. Beat vigorously until dull in color. Pour into buttered pan.