You can store baked bars in an airtight container at room temperature for 10 days or in the fridge for up to 3 weeks. Do not freeze them, because the caramel layer will liquefy when the bars thaw, making them very soft and messy.
- Preheat the oven to 350°F (180°C). Lightly grease a 9 × 13-inch (22.5 × 32.5 cm) pan and line the bottom and sides with parchment paper.
- For the base, place the flour, icing sugar and cocoa powder in the bowl of a food processor with the hazelnuts and pulse until the nuts are finely ground. Add the butter and pulse until the mixture is rough and crumbly, but it does not need to come together.
- Press firmly into the prepared pan and bake for 15 minutes. While the base is cooling on a wire rack, prepare the filling.
- For the filling, place the condensed milk, brown sugar, butter, cream and corn syrup in a medium saucepan over high heat and bring to a boil, stirring constantly with a spatula. Once the mixture reaches a boil, reduce the heat to medium and continue stirring until it reaches 228°F (109°C) on a candy thermometer, about 9 minutes from when it starts to boil. Remove the pan from the heat and stir in the vanilla and salt. Pour over the base and let cool, uncovered, to room temperature, about 2 hours.
- For the topping, place the chocolate and butter in a metal bowl set over a saucepan filled with 1 inch (2.5 cm) of barely simmering water, stirring until melted and smooth. Pour over the cooled caramel and spread or swirl the pan so that the caramel is coated evenly. Sprinkle the chopped hazelnuts over the chocolate and chill, uncovered, until set, about 2 hours. Slice into individual squares once chilled.