For the shortbread crust:
12 ounces (3 sticks) cold unsalted butter, diced, plus more for greasing the baking dish
15 ounces all-purpose flour
3 ounces sugar
¾ teaspoon kosher salt
For the caramel layer:
10 ounces sugar
4 ounces water
3 ounces heavy cream
4 ounces (1 stick) unsalted butter, diced
1/8 teaspoon fleur de sel
For the chocolate layer:
6 ounces semisweet chocolate, chopped or scant 1 cup semisweet chocolate chips
3 ounces unsalted (¾ stick) butter, diced
Make the shortbread crust:
Preheat the oven to 350˚F. Butter a 9×13-inch baking dish and line with parchment paper. Combine all of the ingredients in a mixer fitted with a paddle attachment and mix on low for 8 to 9 minutes, or until the dough comes together. Transfer the dough to the baking dish and press into an even layer. Bake for 35 to 40 minutes, or until golden brown. Let cool to room temperature.
Make the caramel layer:
Combine the sugar and water in a small, heavy saucepan. Bring to a boil, brush down the sides of the pan with water, and boil for 11 to 13 minutes, or until caramelized. The sugar will be fragrant and a deep amber color when it is caramelized. Remove the pan from the heat and dip the bottom into an ice water bath for a second or two. Slowly stir in the cream and butter and then stir in the fleur de sel. Pour the caramel over the shortbread and spread evenly. Let cool to room temperature.
Make the chocolate layer:
Combine the chocolate and butter in a medium bowl, place the bowl over a small pan of simmering water, and heat, stirring frequently, until melted. Pour the chocolate mixture over the caramel layer and spread evenly. Let cool for a couple of hours, or until set.
Using the parchment paper, lift the bars out of the pan and transfer them to a cutting board. Cut into portions and serve.
Makes 24 bars. Inspired by a certain well-known and well-loved candy bar: Twix. For a professional-looking presentation, cut the bars using a hot knife and wipe it clean between cuts. Bars keep for several days in a tightly sealed container in a cool, dry place.