Melt 1/2 cup butter. Add 3 squares unsweetened chocolate, remove from heat. Stir in 1 cup sugar, 1/3 cup dark Karo syrup, 1/4 cup unsifted flour, 1 teaspoon vanilla, 1/4 teaspoon salt and 3 eggs, slightly beaten. Pour in crust. Bake at 350 degrees for 30-35 minutes until center is firm.