1. Preheat the oven to 350 °F (180 °C). Grease an 8-inch (20 cm) square pan and line it with parchment paper so that the paper comes up the sides.
2. Beat the butter and sugar by hand and beat in the egg yolk. Add the flour and salt and stir until the mixture becomes rough and crumbly (it will not come together like a dough). Press this crumble into the prepared pan and bake for about 15 minutes, until it browns a little at the edges. Cool while making the caramel layer (the base does not have to be completely cooled for this.)
Caramel and Assembly
1. For the caramel, measure the water, sugar and condensed milk into a medium saucepot and have all of the other ingredients measured and on hand. Bring the sugar mixture up to a boil on high heat and stirring constantly with a silicone spatula. Once it reaches a boil add the corn syrup and reduce the heat to medium, stirring constantly (this is to prevent the sugars from boiling over) until it reads 230 °F (110 °C) on a candy thermometer, about 7 minutes (the caramel will just be starting to colour a bit). At this point stir in the butter (the temperature will drop by at least 5 °F (3 °C), and then, still on medium heat, stir constantly until the caramel reaches 239 °F (115 °C), 3-4 minutes – at this point the caramel will be a pale brown (lighter than peanut butter colour). Stir in the vanilla and salt and pour this over the shortbread base. Let this cool completely, about 3 hours.
2. Prepare the bars for dipping into chocolate. Lift the base out using the parchment, peel it away and cut the square into 3 strips, and then cut each of those into 10 slender bars. Arrange these on 2 parchment-lined baking trays.
3. To temper the chocolate, you will need a marble board (or a granite or other stone countertop, or a stainless steel counter will also do). Place all of the chopped chocolate in a metal bowl and set this over a pot of barely simmering water (no bubbles visible), stirring gently until melted and the temperature reaches 104 -113 °F (40-45 °C). Pour two thirds of the chocolate onto your marble surface and set bowl with the remaining chocolate off to the side on a towel (away from heat and not on the marble).
4. Using two putty knives, or a palette knife & bench scraper, spread out the chocolate into a thin layer and use your tools to push the chocolate back into the centre of the board, scarping your tools to clean them of the chocolate at each push. This action of moving the chocolate plus the cooling property of the marble will lower the temperature of the chocolate. Keep repeating this process or spreading and scraping in until a temperature of 81 °F (27 °C) is achieved.
5. Using truffle forks or a pair of small or narrow tongs, dip each cookie finger in the chocolate to completely coat it and then shake it vigorously to remove excess chocolate. Place the finger on the baking tray to set while repeating with the remaining fingers. Chill the bars in the fridge for 3-5 minutes (this final “cure” ensures the chocolate will hold its set & shine), then remove from the fridge and store in an airtight container in a cool, dry place (not refrigerated).