- With the rack in the middle position, preheat the oven to 400°F (200°C). Butter an 8-inch (20 cm) square pan and line with a strip of parchment paper, letting it hang over two sides.
- In a bowl, combine the flour and salt.
- In another bowl, cream the butter, sugar and vanilla with an electric mixer. Add the egg yolks one at a time and mix until smooth. Using a wooden spoon, add the dry ingredients. The mixture will be crumbly. With the tips of your fingers, press the mixture into the bottom of the prepared pan.
- Bake for 12 to 14 minutes or until golden. Let cool for 30 minutes.
- In a pot over medium heat, bring the sugar, water, condensed milk and corn syrup to a boil. Cook until a candy thermometer reads 230°F (110°C), stirring a few times.
- Remove the pot from the heat. Add the butter, vanilla and salt. Mix well. Bring back to a boil over medium heat and cook until a candy thermometer reads 239°F (115°C), stirring a few times. Immediately remove from the heat.
- Using a spatula, spread the caramel evenly over the cooled shortbread. Let cool, then refrigerate until completely chilled, about 2 hours.
- Line a baking sheet with a silicone mat or parchment paper.
- On a work surface, unmould the caramel-covered shortbread. Cut the sides slightly to straighten them out. Cut into two equal rectangles. Cut each rectangle into 9 rectangular bars, each about ¾ inches (2 cm) wide for a total of 18 bars. Place the bars on the baking sheet. Refrigerate for 15 minutes.
- Place the cooled chocolate in a tall, narrow container (such as a measuring cup).
- Dip half of each bar into the chocolate and shake to remove any excess. Return the bars to the baking sheet and freeze for 10 minutes or until the chocolate has set. Dip the other half of each bar in the chocolate. Reheat the chocolate in a double boiler or microwave oven as needed. Using the tip of a butter knife, drizzle more chocolate over the surface of the bars, if desired. Let set completely at room temperature, about 3 hours. The bars will keep for 2 weeks in an airtight container at room temperature.
Save any excess melted chocolate. Once it hardens, you will be able to reuse it in other recipes.