Preheat the oven to 350 degrees Fahrenheit and line an 8"x8" square cake pan with foil. Make sure the foil hang over the pan so that you can pop the bars out of the pan later.
First make the shortbread layer. Combine the flour, granulated sugar and salt in a medium mixing bowl. Pour the butter into the dry mixture and store to combine with a wooden spoon until all the dry ingredients are moistened. Tip the dough into the foil lined pan and press it into the pan with your fingers. Make sure it evenly covers the pan and that you press it into the corners. Poke the shortbread with a fork all over, spacing your pokes about a inch apart.
When the shortbread is cool, make the caramel. You're going to want to use a candy thermometer to watch the temperature. Combine all the ingredients for the caramel in a medium sized heavy-bottomed pot and heat over medium-low heat. Stir the mixture frequently and cook until it reaches 236 degrees Fahrenheit. This took about 20 minutes for me. It might take a little more or less for you, just watch for 236.
Once the mixture reaches 236 degrees Fahrenheit, remove it from the heat, pour it over the shortbread, and spread it quickly and evenly over the cookie base.
Let the caramel cool completely. This will take at least an hour and a half.
Once the caramel is completely cooled and set, prepare the chocolate. Place the chocolate in a heat proof bowl, over a simmering pot of water. It's important that the bowl fits in the pot but doesn't touch the simmering water. Stir the chocolate with a rubber spatula until it's all melted. Pour the chocolate over the caramel layer and spread it evenly.
Refrigerate the shortbread until the chocolate is set, about 10-15 minutes.
Once the chocolate is set, remove the shortbread from the pan, using the overhanging foil to lift the shortbread from the pan. Use a serrated knife to cut the shortbread into 20 pieces. I usually slice them in half and then slice each half across into 10, 4 inch, finger-like pieces. Sprinkle with crunchy sea salt and enjoy!