Line an 8-inch square baking pan with aluminum foil. Grease the foil with butter and set aside. Combine sugar, heavy cream, butter, 1/2 teaspoon salt and corn syrup in a heavy-bottomed, medium saucepan and turn the heat to low. Cook, stirring, just until the sugar dissolves, then cook until the mixture measures 248ºF on a candy thermometer. Early in the cooking process and using a pastry brush, wash down the sides of the pan with water to ensure that the sugar crystals have not "jumped" up on the sides of the pan. When the caramel has come to temperature, it is ready. (A small amount of it will form a firm ball if dropped into a glass of cold water. A calibrated candy thermometer is easier and more accurate though.) Once it reaches the appropriate temperature remove pan from heat and add the dulce de leche, the remaining 1/2 teaspoon of salt and balsamic vinegar. Gently stir to combine and pour into prepared pan; stirring to aggressively will cause air bubbles to form. Let the caramel sit in the pan overnight. The next day remove block of caramel from pan and cut into squares. Wrap each square in waxed paper, twisting the ends of the paper. The candies can be kept for weeks and will develop flavor as the time passes.