Before starting the caramel, wash and dry the apples. Skewer the center of each apple with a popsicle stick and set aside.
Add the sugar, butter, syrup, milk and salt in a heavy sauce pan. Cook over low heat and stir frequently as the mixture thickens. Stir constantly as it darkens. Remove from heat when your candy thermometer reaches 250° (firm ball stage). Beat in vanilla.
Working quickly, dip the skewered apples in the caramel, coating as well as possible. Using a spoon, remove any excess caramel on the bottoms before setting on a greased baking sheet. Continue the process until all of the apples are coated. If desired, sprinkle the tops of each apple with a little extra sea salt.
NOTE: Towards the end, you may have to scoop the caramel and spread it over the apples. These ones won't look as pretty, but they will taste just as great! ;)