Pre-heat oven to 350 degrees and line a 9-by-9-inch pan with parchment paper with the edges of the parchment hanging over the sides. This will make removing the bars easier.
In a large bowl combine the butter, sugar and salt with a fork. Add the yolk and combine with the fork. Add the flour and using your hands, combine the dough until its flaky. Transfer the dough into the parchment lined pan and press down with your hands or the bottom of a measuring cup. Refrigerate this for 30 minutes. Once chilled, bake for 25 minutes until lightly browned.
Once baked, remove and set aside to cool.
Meanwhile, in a pot combine the topping ingredients; butter, brown sugar, corn syrup, salt, sugar and cream. Bring to a boil and make sure all the sugar is dissolved. Stir for about 5-10 minutes until its thick and bubbly. If you have a candy thermometer - the caramel should be about 248 degrees. Remove from heat, stir in vanilla extract and pour on top of the shortbread.
Refrigerate for 2-4 hours. Then sprinkle with the sea salt flakes and serve!